Add missing ( to ExternalFunctionItem
>
> _ExternalFunctionItem_ :\
> &nbsp;&nbsp; `fn` [IDENTIFIER]&nbsp;[_Generics_]<sup>?</sup>\
-> &nbsp;&nbsp; `(` _NamedFunctionParameters_<sup>?</sup> | _NamedFunctionParametersWithVariadics_ ) `)`\
+> &nbsp;&nbsp; `(` ( _NamedFunctionParameters_<sup>?</sup> | _NamedFunctionParametersWithVariadics_ ) `)`\
> &nbsp;&nbsp; [_FunctionReturnType_]<sup>?</sup> [_WhereClause_]<sup>?</sup> `;`
>
> _NamedFunctionParameters_ :\
