Clean up E0670 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0670.md

@@ -1,6 +1,6 @@
 Rust 2015 does not permit the use of `async fn`.
 
-Example of erroneous code:
+Erroneous code example:
 
 ```compile_fail,E0670
 async fn foo() {}