Correct string in unit tests

[caphrim007]

SUMMARY

Just a simple cleanup. nothing important.

ISSUE TYPE

• Bugfix Pull Request

COMPONENT NAME

bigip modules

ANSIBLE VERSION

ansible 2.7.0.dev0
  config file = None
  configured module search path = ['/root/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /usr/local/lib/python3.6/site-packages/ansible
  executable location = /usr/local/bin/ansible
  python version = 3.6.6 (default, Jul 17 2018, 11:12:33) [GCC 6.3.0 20170516]

ADDITIONAL INFORMATION

[ansibot]

cc @Qalthos @ganeshrn @gdpak @justjais @rcarrillocruz @trishnaguha
click here for bot help

[ansibot]

The test ansible-test sanity --test validate-modules [explain] failed with 5 errors:

lib/ansible/modules/network/f5/bigip_profile_dns.py:0:0: E309 version_added for new option (cache_name) should be 2.7. Currently 0.0
lib/ansible/modules/network/f5/bigip_profile_dns.py:0:0: E309 version_added for new option (enable_dns_cache) should be 2.7. Currently 0.0
lib/ansible/modules/network/f5/bigip_profile_dns.py:0:0: E322 "enable_cache" is listed in the argument_spec, but not documented in the module
lib/ansible/modules/network/f5/bigip_profile_dns.py:0:0: E323 "enable_dns_cache" is listed in DOCUMENTATION.options, but not accepted by the module
lib/ansible/modules/network/f5/bigip_profile_dns.py:0:0: E325 argument_spec for "enable_cache" defines type="bool" but documentation does not

click here for bot help