Go with "constant items" for _ in index

In the keywords table in the syntax index, every entry except the
one for constant items for `_` consists entirely of a link body.  For
that one, though, we had left "unnamed" out of the link body.
Visually, that just stands out too much.

We could just put "unnamed" in the link body, but most of the other
entries aren't really that specific.  It seems OK to just say
"constant items" for this, so let's do that.

Author: traviscross

src/syntax-index.md

@@ -6,7 +6,7 @@ This appendix provides an index of tokens and common forms with links to where t
 
 | Keyword       | Usage |
 |---------------|-------|
-| `_`           | [wildcard pattern], [inferred const], [inferred type], [placeholder lifetime], unnamed [constant items], [extern crate], [use declarations], [destructuring assignment] |
+| `_`           | [wildcard pattern], [inferred const], [inferred type], [placeholder lifetime], [constant items], [extern crate], [use declarations], [destructuring assignment] |
 | `abstract`    | [reserved keyword] |
 | `as`          | [extern crate][items.extern-crate.as], [use declarations][items.use.forms.as], [type cast expressions], [qualified paths] |
 | `async`       | [async functions], [async blocks], [async closures] |