fix(client): restore --cluster=dev as apps:create default

In refactoring the Deis CLI docstrings, we broke the default cluster
for apps:create. This was pointed out by smoke_test.go.

Author: mboersma

client/deis.py

@@ -415,7 +415,7 @@ def apps_create(self, args):
 
         Options:
           --cluster=<cluster>
-            target cluster to host application (default: dev).
+            target cluster to host application [default: dev].
           --no-remote
             do not create a `deis` git remote.
         """