Revise list item for type alias definitions

In this list, we call many things "definitions" to distinguish that
we're only talking about items here.  While it is more clear to say
"type alias definitions" rather than "type definitions" when referring
to type aliases, let's keep "definitions" in there for now as we use
this terminology in other places, especially related to associated
type definitions, which are spiritually similar.

Author: traviscross

src/items.md

@@ -46,7 +46,7 @@ There are several kinds of items:
 * [`extern crate` declarations]
 * [`use` declarations]
 * [function definitions]
-* [type aliases]
+* [type alias definitions]
 * [struct definitions]
 * [enumeration definitions]
 * [union definitions]
@@ -93,5 +93,5 @@ See [item scopes] for information on the scoping rules of items.
 [struct definitions]: items/structs.md
 [trait definitions]: items/traits.md
 [traits]: items/traits.md
-[type aliases]: items/type-aliases.md
+[type alias definitions]: items/type-aliases.md
 [union definitions]: items/unions.md