Fix grammar for tuple struct patterns.
@@ -556,7 +556,7 @@ A struct pattern is refutable when one of its subpatterns is refutable.
> **<sup>Syntax</sup>**\
> _TupleStructPattern_ :\
-> &nbsp;&nbsp; [_PathInExpression_] `(` _TupleStructItems_ `)`
+> &nbsp;&nbsp; [_PathInExpression_] `(` _TupleStructItems_<sup>?</sup> `)`
>
> _TupleStructItems_ :\
> &nbsp;&nbsp; &nbsp;&nbsp; [_Pattern_]&nbsp;( `,` [_Pattern_] )<sup>\*</sup> `,`<sup>?</sup>\
