gh-100546: Remove incorrect positional-only marker from eval (GH-100547)

All the arguments are positional-only.

The current status after GH-99476 seems to be to not use positional-only
markers in documentation, hence I've simply removed it.
(cherry picked from commit 71159a8)

Co-authored-by: Shantanu <[email protected]>

Author: miss-islington

Doc/library/functions.rst

@@ -512,7 +512,7 @@ are always available.  They are listed here in alphabetical order.
 
 .. _func-eval:
 
-.. function:: eval(expression, /, globals=None, locals=None)
+.. function:: eval(expression, globals=None, locals=None)
 
    The arguments are a string and optional globals and locals.  If provided,
    *globals* must be a dictionary.  If provided, *locals* can be any mapping