Fix a typo in 'attach_volume' docstring

Removed redundant 's'

Change-Id: I01b5132c86c64bb6396b515aace0f022315a9aff
diff --git a/tempest/api/volume/base.py b/tempest/api/volume/base.py
index 90dc7f4..e39d40f 100644
--- a/tempest/api/volume/base.py
+++ b/tempest/api/volume/base.py
@@ -172,7 +172,7 @@
         client.wait_for_resource_deletion(volume_id)
 
     def attach_volume(self, server_id, volume_id):
-        """Attachs a volume to a server"""
+        """Attach a volume to a server"""
         self.servers_client.attach_volume(
             server_id, volumeId=volume_id,
             device='/dev/%s' % CONF.compute.volume_device_name)