Fix a typo in nova_compute documentation string

Without this patch, ansible-doc was failing this way:

$ ansible-doc nova_compute
Traceback (most recent call last):
  File "/home/francois/WORK/dev/ansible/bin/ansible-doc", line 324, in <module>
    main()
  File "/home/francois/WORK/dev/ansible/bin/ansible-doc", line 316, in main
    text += get_man_text(doc)
  File "/home/francois/WORK/dev/ansible/bin/ansible-doc", line 112, in get_man_text
    desc = " ".join(opt['description'])
KeyError: 'description'