doc: slight consistency fix

Others in this list are Capitalized, so do it here too.

Author: tshepang

src/doc/guide.md

@@ -3025,7 +3025,7 @@ markers.
 Rust provides six attributes to indicate the stability level of various
 parts of your library. The six levels are:
 
-* deprecated: this item should no longer be used. No guarantee of backwards
+* deprecated: This item should no longer be used. No guarantee of backwards
   compatibility.
 * experimental: This item was only recently introduced or is otherwise in a
   state of flux. It may change significantly, or even be removed. No guarantee