Clean up E0054

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0054.md

@@ -1,5 +1,6 @@
-It is not allowed to cast to a bool. If you are trying to cast a numeric type
-to a bool, you can compare it with zero instead:
+It is not allowed to cast to a bool.
+
+Erroneous code example:
 
 ```compile_fail,E0054
 let x = 5;
@@ -8,6 +9,9 @@ let x = 5;
 let x_is_nonzero = x as bool;
 ```
 
+If you are trying to cast a numeric type to a bool, you can compare it with
+zero instead:
+
 ```
 let x = 5;