Clean up E0069 long explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0069.md

@@ -1,5 +1,7 @@
 The compiler found a function whose body contains a `return;` statement but
-whose return type is not `()`. An example of this is:
+whose return type is not `()`.
+
+Erroneous code example:
 
 ```compile_fail,E0069
 // error