Sync `latex_symbols.jl` url in two places

Follow-up to  #11399 and  #11400.

Make sure the comment at the top of

https://github.com/ipython/ipython/blob/master/IPython/core/latex_symbols.py

points to the file actually used in

- https://github.com/ipython/ipython/edit/master/tools/gen_latex_symbols.py

tools/gen_latex_symbols.py

@@ -1,87 +1,87 b''
 # coding: utf-8
 
 # This script autogenerates `IPython.core.latex_symbols.py`, which contains a
 # single dict , named `latex_symbols`. The keys in this dict are latex symbols,
 # such as `\\alpha` and the values in the dict are the unicode equivalents for
 # those. Most importantly, only unicode symbols that are valid identifiers in
 # Python 3 are included. 
 
 # 
 # The original mapping of latex symbols to unicode comes from the `latex_symbols.jl` files from Julia.
 
 from pathlib import Path
 
 # Import the Julia LaTeX symbols
 print('Importing latex_symbols.js from Julia...')
 import requests
 url = 'https://raw.githubusercontent.com/JuliaLang/julia/master/stdlib/REPL/src/latex_symbols.jl'
 r = requests.get(url)
 
 
 # Build a list of key, value pairs
 print('Building a list of (latex, unicode) key-value pairs...')
 lines = r.text.splitlines()
 
 prefixes_line = lines.index('# "font" prefixes')
 symbols_line = lines.index('# manual additions:')
 
 prefix_dict = {}
 for l in lines[prefixes_line + 1: symbols_line]:
     p = l.split()
     if not p or p[1] == 'latex_symbols': continue
     prefix_dict[p[1]] = p[3]
 
 idents = []
 for l in lines[symbols_line:]:
     if not '=>' in l: continue # if it's not a def, skip
     if '#' in l: l = l[:l.index('#')] # get rid of eol comments
     x, y = l.strip().split('=>') 
     if '*' in x: # if a prefix is present substitute it with its value
         p, x = x.split('*')
         x = prefix_dict[p][:-1] + x[1:]
     x, y = x.split('"')[1], y.split('"')[1] # get the values in quotes
     idents.append((x, y))
 
 # Filter out non-valid identifiers
 print('Filtering out characters that are not valid Python 3 identifiers')
 
 def test_ident(i):
     """Is the unicode string valid in a Python 3 identifier."""
     # Some characters are not valid at the start of a name, but we still want to
     # include them. So prefix with 'a', which is valid at the start.
     return ('a' + i).isidentifier()
 
 assert test_ident("α")
 assert not test_ident('‴')
 
 valid_idents = [line for line in idents if test_ident(line[1])]
 
 # Write the `latex_symbols.py` module in the cwd
 
-s = """# encoding: utf-8
+s = f"""# encoding: utf-8
 
 # DO NOT EDIT THIS FILE BY HAND.
 
 # To update this file, run the script /tools/gen_latex_symbols.py using Python 3
 
 # This file is autogenerated from the file:
-# https://raw.githubusercontent.com/JuliaLang/julia/master/base/latex_symbols.jl
+# {url}
 # This original list is filtered to remove any unicode characters that are not valid
 # Python identifiers.
 
-latex_symbols = {\n
+latex_symbols = {{\n
 """
 for line in valid_idents:
     s += '    "%s" : "%s",\n' % (line[0], line[1])
 s += "}\n"
 
 s += """
 
 reverse_latex_symbol = { v:k for k,v in latex_symbols.items()}
 """
 
 fn = Path("..", "IPython", "core", "latex_symbols.py")
 print("Writing the file: %s" % str(fn))
 fn.write_text(s, encoding="utf-8")