docs(firestore): remove duplicated word in README (#9297)

Removed redundant text

Author: PaulTR

README.rst

@@ -110,6 +110,6 @@ Next Steps
 ~~~~~~~~~~
 
 -  Read the `Client Library Documentation`_ for Google Cloud Firestore API
-   API to see other available methods on the client.
+   to see other available methods on the client.
 -  Read the `Product Documentation`_ to learn
    more about the product and see How-to Guides.