chore: fix typo in retain_force method doc

src/map.rs

@@ -2686,7 +2686,7 @@ where
     ///
     /// In other words, remove all pairs `(k, v)` such that `f(&k,&v)` returns `false`.
     ///
-    /// This method always deletes any key/value pair that `f` returns `false` for, even if if the
+    /// This method always deletes any key/value pair that `f` returns `false` for, even if the
     /// value is updated concurrently. If you do not want that behavior, use [`HashMap::retain`].
     ///
     /// # Examples