Fix: typo in E0751 error explanation

Corrected a grammatical error in the explanation for E0751. Changed "exists" to "exist" to improve clarity and ensure proper grammar in the error message.

compiler/rustc_error_codes/src/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, both cannot exists at the same time.
+for the given types. Therefore, both cannot exist at the same time.