Improve wording

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0751.md

@@ -9,4 +9,4 @@ impl !MyTrait for i32 { } // error!
 ```
 
 Negative implementations are a promise that the trait will never be implemented
-for the given types. Therefore, you can't have both at the same time!
+for the given types. Therefore, both cannot exists at the same time.