docs: refer to 'embed_kernel' in 'start_kernel's docstring

When 'start_kernel' was added in a10986ac3 (add IPython.start_kernel,
2013-07-04), its docstring was probably copied from that of
'start_ipython', such that instead of referencing its counterpart
'embed_kernel', it references 'start_ipython's counterpart 'embed'.

Correctly refer to 'embed_kernel'.

IPython/__init__.py

@@ -132,7 +132,7 b' def start_kernel(argv=None, **kwargs):'
 
     `start_kernel()` does full, regular IPython initialization,
     including loading startup files, configuration, etc.
-    much of which is skipped by `embed()`.
+    much of which is skipped by `embed_kernel()`.
 
     Parameters
     ----------