Make explicit reference to scrutinee expression in grammar snippet
> **<sup>Syntax</sup>**\
> _MatchExpression_ :\
-> &nbsp;&nbsp; `match` [_Expression_]<sub>_except struct expression_</sub> `{`\
+> &nbsp;&nbsp; `match` Scrutinee `{`\
> &nbsp;&nbsp; &nbsp;&nbsp; [_InnerAttribute_]<sup>\*</sup>\
> &nbsp;&nbsp; &nbsp;&nbsp; _MatchArms_<sup>?</sup>\
> &nbsp;&nbsp; `}`
>
+>_Scrutinee_ :\
+> &nbsp;&nbsp; [_Expression_]<sub>_except struct expression_</sub>
+>
> _MatchArms_ :\
> &nbsp;&nbsp; ( _MatchArm_ `=>`
> ( [_ExpressionWithoutBlock_][_Expression_] `,`
