Fixed small typo in Virtual methods example README
Example for how to create a base widget which exposes virtual methods that other widgets can override.
In this case, a `BaseButton` class is defined which is a gtk::Button with two additional methods:
-* sync_method`: Synchronous method that updates the button label to `BaseButton sync`, or `BaseButton sync {extra_text}`. This showcases how arguments
+* `sync_method`: Synchronous method that updates the button label to `BaseButton sync`, or `BaseButton sync {extra_text}`. This showcases how arguments
can be passed to virtual methods. The `Option<String>` has to be boxed to be FFI-safe.
* `async_method`: Asynchronous method that updates the button label to `BaseButton async`
