Clean up E0728 explanation

src/librustc_error_codes/error_codes/E0728.md

@@ -1,6 +1,6 @@
-[`await`] has been used outside [`async`] function or block.
+[`await`] has been used outside [`async`] function or [`async`] block.
 
-Erroneous code examples:
+Erroneous code example:
 
 ```edition2018,compile_fail,E0728
 # use std::pin::Pin;
@@ -33,7 +33,7 @@ fn foo() {
 
 [`await`] is used to suspend the current computation until the given
 future is ready to produce a value. So it is legal only within
-an [`async`] context, like an `async fn` or an `async` block.
+an [`async`] context, like an `async` function or an `async` block.
 
 ```edition2018
 # use std::pin::Pin;