Docstring fix for CreateVolumeAttachment class

The command "volume attachment create" has a typo in the docstring.
The docstring says to use "server add volume", but the command is
actually "server volume add". This
change fixes the typo in the docstring.

Task: 46781
Story: 2010401
Change-Id: Ie19a24ead100dd9177669653a7a9997772ef4538

Author: jadonn

openstackclient/volume/v3/volume_attachment.py

@@ -82,7 +82,7 @@ class CreateVolumeAttachment(command.ShowOne):
     the volume to the server at the hypervisor level. As a result, it should
     typically only be used for troubleshooting issues with an existing server
     in combination with other tooling. For all other use cases, the 'server
-    volume add' command should be preferred.
+    add volume' command should be preferred.
     """
 
     def get_parser(self, prog_name):