fix Functor docs

src/func.rs

@@ -52,7 +52,8 @@ pub trait WeakFunctor {
 ///     }
 /// }
 /// ```
-/// Why does it fail to compile? `.map` expects `FnMut` (or we can think of it being `Fn`, doesn't matter here). But what we provide it (`f`) is
+/// Why does it fail to compile? `.map` expects `FnMut` (or we can think of it being `Fn`, doesn't matter here).
+/// But what we provide it (`f`) is only `FnOnce`.
 ///
 /// For Haskell-style Functors, use [`clone_func::CloneFunctor`] instead.
 /// ```