Rollup merge of #73436 - GuillaumeGomez:cleanup-e0670, r=Dylan-DPC

Clean up E0670 explanation

r? @Dylan-DPC

Author: Manishearth

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() {}