Remove extraneous ?s in struct grammar

Havvy · Havvy · commit c20b106a20fc · 2018-02-07T10:30:12.000-08:00
The visibility grammar already has `EMPTY` as an option.
diff --git a/src/items/structs.md b/src/items/structs.md
@@ -25,15 +25,15 @@
 >  
 > _StructField_ :  
 > &nbsp;&nbsp; [_OuterAttribute_]<sup>\*</sup>  
-> &nbsp;&nbsp; [_Visibility_]<sup>?</sup>  
+> &nbsp;&nbsp; [_Visibility_]
 > &nbsp;&nbsp; [IDENTIFIER] `:` [_Type_]  
 >  
 > _TupleFields_ :  
 > &nbsp;&nbsp; _TupleField_ (`,` _TupleField_)<sup>\*</sup> `,`<sup>?</sup>  
 >  
 > _TupleField_ :  
 > &nbsp;&nbsp; [_OuterAttribute_]<sup>\*</sup>  
-> &nbsp;&nbsp; [_Visibility_]<sup>?</sup>  
+> &nbsp;&nbsp; [_Visibility_]
 > &nbsp;&nbsp; [_Type_]  
 
 A _struct_ is a nominal [struct type] defined with the keyword `struct`.

Original file line number	Diff line number	Diff line change
`@@ -25,15 +25,15 @@`
`25`	`25`	`>`
`26`	`26`	`> _StructField_ :`
`27`	`27`	`>    [_OuterAttribute_]<sup>\*</sup>`
`28`		`->    [_Visibility_]<sup>?</sup>`
	`28`	`+>    [_Visibility_]`
`29`	`29`	>    [IDENTIFIER] `:` [_Type_]
`30`	`30`	`>`
`31`	`31`	`> _TupleFields_ :`
`32`	`32`	>    _TupleField_ (`,` _TupleField_)<sup>\*</sup> `,`<sup>?</sup>
`33`	`33`	`>`
`34`	`34`	`> _TupleField_ :`
`35`	`35`	`>    [_OuterAttribute_]<sup>\*</sup>`
`36`		`->    [_Visibility_]<sup>?</sup>`
	`36`	`+>    [_Visibility_]`
`37`	`37`	`>    [_Type_]`
`38`	`38`
`39`	`39`	A _struct_ is a nominal [struct type] defined with the keyword `struct`.