Clarify description of items in syntax index

On the branch, in the syntax index, items were described as
"declarations in a crate".  Some items may be declarations, but many
items are actually definitions.  It's probably better to not use
either term here.  Let's instead say that items are the components of
a crate as that's the language that's used in the items chapter.

Author: traviscross

src/syntax-index.md

@@ -202,7 +202,7 @@ This appendix provides an index of tokens and common forms with links to where t
 
 ## Items
 
-[Items] are declarations in a crate.
+[Items] are the components of a crate.
 
 | Item                          | Usage |
 |-------------------------------|-------|