Fix grammar for unsafe attributes

There were some errors in the formatting and the content of the
grammar for unsafe attributes.  Let's fix those.

Author: traviscross

src/attributes.md

@@ -3,14 +3,12 @@
 
 > **<sup>Syntax</sup>**\
 > _InnerAttribute_ :\
-> &nbsp;&nbsp; `#` `!` `[` _Attr_ `]`
-> &nbsp;&nbsp; `#` `!` `[` `unsafe` `(` _Attr_ `)` `]`
+> &nbsp;&nbsp; &nbsp;&nbsp; `#` `!` `[` _Attr_ `]`\
+> &nbsp;&nbsp; | `#` `!` `[` `unsafe` `(` _Attr_ `)` `]`
 >
 > _OuterAttribute_ :\
-> &nbsp;&nbsp; `#` `[` _Attr_ `]`
-> &nbsp;&nbsp; `#` `[` `unsafe` `(` _Attr_ `)` `]`
-> &nbsp;&nbsp; `#` `!` `[` _Attr_ `]`
-> &nbsp;&nbsp; `#` `!` `[` `unsafe` `(` _Attr_ `)` `]`
+> &nbsp;&nbsp; &nbsp;&nbsp; `#` `[` _Attr_ `]`\
+> &nbsp;&nbsp; | `#` `[` `unsafe` `(` _Attr_ `)` `]`
 >
 > _Attr_ :\
 > &nbsp;&nbsp; [_SimplePath_] _AttrInput_<sup>?</sup>