Change lint doc test

Author: FrancisMurillo

clippy_lints/src/mut_mutex_lock.rs

@@ -22,7 +22,7 @@ declare_clippy_lint! {
     /// let value_mutex = Arc::get_mut(&mut value_rc).unwrap();
     ///
     /// let mut value = value_mutex.lock().unwrap();
-    /// value += 1;
+    /// *value += 1;
     /// ```
     /// Use instead:
     /// ```rust
@@ -32,7 +32,7 @@ declare_clippy_lint! {
     /// let value_mutex = Arc::get_mut(&mut value_rc).unwrap();
     ///
     /// let value = value_mutex.get_mut().unwrap();
-    /// value += 1;
+    /// *value += 1;
     /// ```
     pub MUT_MUTEX_LOCK,
     correctness,