Simplify the grammar for GenericParams

The definition of GenericParams had separate productions for the cases
of zero params and one-or-more params. Merge those two cases, to make
the railroad diagram clearer.

Author: joshtriplett

src/items/generics.md

@@ -3,9 +3,7 @@ r[items.generics]
 
 r[items.generics.syntax]
 ```grammar,items
-GenericParams ->
-      `<` `>`
-    | `<` (GenericParam `,`)* GenericParam `,`? `>`
+GenericParams -> `<` ( (GenericParam `,`)* GenericParam `,`? )? `>`
 
 GenericParam -> OuterAttribute* ( LifetimeParam | TypeParam | ConstParam )