Move the recursion in GenericParams to the right

This makes it easier to read the production from left to right:
a GenericParam, zero or more (comma followed by GenericParam), optional
trailing comma.

Author: joshtriplett

src/items/generics.md

@@ -3,7 +3,7 @@ r[items.generics]
 
 r[items.generics.syntax]
 ```grammar,items
-GenericParams -> `<` ( (GenericParam `,`)* GenericParam `,`? )? `>`
+GenericParams -> `<` ( GenericParam (`,` GenericParam)* `,`? )? `>`
 
 GenericParam -> OuterAttribute* ( LifetimeParam | TypeParam | ConstParam )