Fix the definition of MetaItem and MetaSeq

The current definitions mean that the following are valid attributes:

- `#[foo(,)]`
- `#[foo(, , , , ,)]`

These aren't (intended to be) valid. This fixes the definitions to only allow an EMPTY at the end of a comma-separated list.

Author: mjbshaw

src/attributes.md

@@ -15,12 +15,12 @@
 >    | IDENTIFIER `=` LITERAL  
 >    | IDENTIFIER `(` LITERAL `)`  
 >    | IDENTIFIER `(` _MetaSeq_ `)`  
->    | IDENTIFIER `(` _MetaSeq_ `,` `)`  
+>    | IDENTIFIER `(` _MetaItem_ `,` _MetaSeq_ `)`  
 >   
 > _MetaSeq_ :  
 >       EMPTY  
 >    | _MetaItem_  
->    | _MetaSeq_ `,` _MetaItem_  
+>    | _MetaItem_ `,` _MetaSeq_
 
 Any item declaration may have an _attribute_ applied to it. Attributes in Rust
 are modeled on Attributes in ECMA-335, with the syntax coming from ECMA-334