Fix typo in extension example project (#1002)

Minor typo fix.

Author: roschler

examples/extension/README.md

@@ -21,7 +21,7 @@ An example project to show how to run 🤗 Transformers in a browser extension.
 
 1. Add the extension to your browser. To do this, go to `chrome://extensions/`, enable developer mode (top right), and click "Load unpacked". Select the `build` directory from the dialog which appears and click "Select Folder".
 
-1. That's it! You should now be able to open the extenion's popup and use the model in your browser!
+1. That's it! You should now be able to open the extension's popup and use the model in your browser!
 
 ## Editing the template