Clean up E0600 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0600.md

@@ -1,6 +1,6 @@
 An unary operator was used on a type which doesn't implement it.
 
-Example of erroneous code:
+Erroneous code example:
 
 ```compile_fail,E0600
 enum Question {