Update README.md

Removed typo.

Author: Ephasme

README.md

@@ -59,7 +59,7 @@ var vm = new Vue({
 
 **About the cancelCallback**: This callback is actually an error handler that
 is called if the read/write doesn't succeed. You can of course also use it with
-`asObject: false` and apply it to arrays. The firebase doc stateees:
+`asObject: false` and apply it to arrays. The firebase doc states:
 
 > We can pass an optional cancel callback to our event subscription that will
 > be called if the read is ever cancelled. A read would be cancelled if the app