clean up E0718 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0718.md

@@ -1,7 +1,6 @@
-This error indicates that a `#[lang = ".."]` attribute was placed
-on the wrong type of item.
+A `#[lang = ".."]` attribute was placed on the wrong type of item.
 
-Examples of erroneous code:
+Erroneous code example:
 
 ```compile_fail,E0718
 #![feature(lang_items)]