Indicate list of primitive types is non-exhaustive

In the syntax index, in the type expressions table, we list some
primitive types but not all of them.  Let's use the ellipses to
indicate that the list is not exhaustive.

Author: traviscross

src/syntax-index.md

@@ -226,7 +226,7 @@ This appendix provides an index of tokens and common forms with links to where t
 
 | Type                                  | Usage |
 |---------------------------------------|-------|
-| `bool`, `u8`, `f64`, `char`, `str`    | [primitive types] |
+| `bool`, `u8`, `f64`, `str`, `…`       | [primitive types] |
 | `for<…>`                              | [higher-ranked trait bounds] |
 | `T: U`                                | [trait bounds] |
 | `T: 'a`                               | [lifetime bounds] |