Fix check_service_client_function doc typo

This commit fixes a doc typo of check_service_client_function to_utf
parameter in lib.tests.services.base.BaseServiceTest class. This
parameter is used for a response body not for a request body.

Change-Id: Ife265758916c4272558b9d7b6652111053bca9bb
diff --git a/tempest/tests/lib/services/base.py b/tempest/tests/lib/services/base.py
index 90c9f63..778c966 100644
--- a/tempest/tests/lib/services/base.py
+++ b/tempest/tests/lib/services/base.py
@@ -40,7 +40,7 @@
                function.
         :param body: Expected response body returned by the service client
                function.
-        :param to_utf: Whether to use UTF-8 encoding for request.
+        :param to_utf: Whether to use UTF-8 encoding for response.
         :param status: Expected response status returned by the service client
                function.
         :param headers: Expected headers returned by the service client