Fix slice types syntax in syntax index

On the branch, in the syntax index, we had notated a slice type so as
to include a type and then ellipses, but it's more correct for a slice
type to just have a type in the square brackets, so let's do that.

Author: traviscross

src/syntax-index.md

@@ -234,7 +234,7 @@ This appendix provides an index of tokens and common forms with links to where t
 | `T: ?Sized`                           | [relaxed trait bounds] |
 | `[Type; len]`                         | [array types] |
 | `(Type, …)`                           | [tuple types] |
-| `[Type, …]`                           | [slice types] |
+| `[Type]`                              | [slice types] |
 | `(Type)`                              | [parenthesized types] |
 | `impl Trait`                          | [impl trait types], [anonymous type parameters] |
 | `dyn Trait`                           | [trait object types] |