Fix a few grammatical errors in docs

This patch fixes a few grammatical errors in documentation.

Change-Id: I5aa92537165aa452649f71e94d6d569011b78fa4
diff --git a/doc/source/plugin.rst b/doc/source/plugin.rst
index d34023f..285ad5d 100644
--- a/doc/source/plugin.rst
+++ b/doc/source/plugin.rst
@@ -213,7 +213,7 @@
 * **client_names**: Name of the classes that implement service clients in the
   service clients module.
 
-Example usage of the the service clients in tests::
+Example usage of the service clients in tests::
 
    # my_creds is instance of tempest.lib.auth.Credentials
    # identity_uri is v2 or v3 depending on the configuration