Correct typo in unsaved quit error message

src/main.rs

@@ -368,7 +368,7 @@ impl State {
                 if self.last_saved == *self.buffer.as_ref() {
                     self.quit = true;
                 } else {
-                    return Err("Buffer is unsaved (add ! to override".into());
+                    return Err("Buffer is unsaved (add ! to override)".into());
                 }
             }
             "w" => return self.write_command(command, args),