fix(client): remove nonexistant certs:update

Author: Joshua Anderson

client/deis.py

@@ -1093,7 +1093,6 @@ def certs(self, args):
 
         certs:list            list SSL certificates for an app
         certs:add             add an SSL certificate to an app
-        certs:update          update an existing certifcate for an app
         certs:remove          remove an SSL certificate from an app
 
         Use `deis help [command]` to learn more.