Clean up E0734 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0734.md

@@ -1,6 +1,6 @@
 A stability attribute has been used outside of the standard library.
 
-Erroneous code examples:
+Erroneous code example:
 
 ```compile_fail,E0734
 #[rustc_deprecated(since = "b", reason = "text")] // invalid