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

@@ -58,18 +58,18 b' 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])