Rollup merge of rust-lang#74835 - GuillaumeGomez:cleanup-e0734, r=jyn514

Clean up E0734 explanation

r? @Dylan-DPC

Author: Manishearth

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