update README with new hook usage

Author: rmorshea

README.md

@@ -45,13 +45,10 @@ IDOM can be used to create a simple slideshow which changes whenever a user clic
 import idom
 
 @idom.element
-async def Slideshow(self, index=0):
-
-    async def next_image(event):
-        self.update(index + 1)
-
+async def Slideshow():
+    index, set_index = idom.hooks.use_state(0)
     url = f"https://picsum.photos/800/300?image={index}"
-    return idom.html.img({"src": url, "onClick": next_image})
+    return idom.html.img({"src": url, "onClick": lambda event: set_index(index + 1)})
 
 server = idom.server.sanic.PerClientState(Slideshow)
 server.daemon("localhost", 8765).join()