Fix typo (vercel#471)

Author: Primigenus

README.md

@@ -225,7 +225,7 @@ Above `Router` object comes with the following API:
 
 The second `as` parameter for `push` and `replace` is an optional _decoration_ of the URL. Useful if you configured custom routes on the server.
 
-_Note: in order to programmatically change the route without triggering navigation and component-fetching, use `props.url.push` and `props.url.replace` withing a component_
+_Note: in order to programmatically change the route without triggering navigation and component-fetching, use `props.url.push` and `props.url.replace` within a component_
 
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