Update README.md (reactjs#215)

Fixed typo

Author: Sepehr Sobhani

README.md

@@ -120,7 +120,7 @@ By default only the current active tab will be rendered to DOM. If set to `true`
 
 > default: `undefined`
 
-This event handler is called every time a tab is about to changed. It will be called with the `index` that will be changed to, the `lastIndex` which was selected before and the underlying `event` which is usually either a `keydown` or `click` event. When `index` and `lastIndex` are equal it means the user clicked on the currently active tab.
+This event handler is called every time a tab is about to change. It will be called with the `index` that it will be changed to, the `lastIndex` which was selected before and the underlying `event` which is usually either a `keydown` or `click` event. When `index` and `lastIndex` are equal it means the user clicked on the currently active tab.
 
 The callback can optionally return `true` to cancel the change to the new tab.