Fix documentation example for the fail module

The example for the fail module doesn't work:
  http://www.ansibleworks.com/docs/modules.html#fail

The current text shows:
    - fail: msg="The system may not be provisioned according to the CMDB status."
      when: "{{ cmdb_status }} != 'to-be-staged'"

The "when" documentation indicates that the argument is already a Jinja2
expression:
  http://www.ansibleworks.com/docs/playbooks_conditionals.html#the-when-statement

Thus, the following is
      when: cmdb_status != "to-be-staged"

is preferred even though the following could work but generates a
deprecation warning:
      when: {{cmdb_status != "to-be-staged"}}
Merge pull request #5436 from jkleckner/fix-fail-doc-example

Fix documentation example for the fail module