[docs] small errors in grammar definition

[Gerben]

Hi, something looks wrong in the syntax definition of function definitions (python 3). At least, there is one unmatched closing parethesis at the end of parameter_list, but I think somebody should check the whole thing.
See it annotated here: https://via.hypothes.is/https://docs.python.org/3/reference/compound_stmts.html#function-definitions
Cheers,

Gerben