IMO better borrow_mut() documentation on RefCell

Previous borrow() is enough to make borrow_mut() panic, no need to have borrow_mut() twice. [This](http://is.gd/woKKAW)

src/libcore/cell.rs

@@ -417,7 +417,7 @@ pub fn borrow<'a>(&'a self) -> Ref<'a, T> {
     ///
     /// let result = thread::spawn(move || {
     ///    let c = RefCell::new(5);
-    ///    let m = c.borrow_mut();
+    ///    let m = c.borrow();
     ///
     ///    let b = c.borrow_mut(); // this causes a panic
     /// }).join();