Clean up E0727 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0727.md

@@ -1,6 +1,6 @@
 A `yield` clause was used in an `async` context.
 
-Example of erroneous code:
+Erroneous code example:
 
 ```compile_fail,E0727,edition2018
 #![feature(generators)]