Fixing grammar mistake
Fixing grammar mistake

The description of the param 'content_type' doesn't read well,
I think 'Content-Type of the response' is better.

Change-Id: Idfdedf2cfc1abf6ab6f6980114c6d7d00c557afc

openstack/mistral,StackStorm/mistral,StackStorm/mistral,openstack/mistral