Fix a typo

Author: vrana

CONTRIBUTING.md

@@ -79,7 +79,7 @@ Our code is our lifeblood so maintaining CesiumJS's high code quality is importa
   - If you added new identifiers to the CesiumJS API:
     - Update [CHANGES.md](CHANGES.md).
     - Include reference documentation with code examples. Follow the [Documentation Guide](Documentation/Contributors/DocumentationGuide/README.md).
-    - If the change is significant, add a new [Sandcastle](https://sandcastle.cesium.com) example or extend and existing one.
+    - If the change is significant, add a new [Sandcastle](https://sandcastle.cesium.com) example or extend an existing one.
   - If you added third-party libraries, including new version of existing libraries, update [LICENSE.md](LICENSE.md). Mention it in [CHANGES.md](CHANGES.md). If you plan to add a third-party library, start a [GitHub issue](https://github.com/CesiumGS/cesium/issues/new) discussing it first.
 
 ## Code of Conduct