Doc uses alias instead of aliases
Doc uses alias instead of aliases

The overview_policies doc makes reference to an `alias` option when in
fact the option is `aliases`.
The sample storage policy snippet is correct, it's just incorrect when
listing the possible options.

This change changes the listed option to `aliases`.

Change-Id: Iddf0f19f4d50819ff6abd46e6a1156dc8e4a451d

swiftstack/swift,openstack/swift,smerritt/swift,matthewoliver/swift,matthewoliver/swift,tipabu/swift,tipabu/swift,nadeemsyed/swift,tipabu/swift,openstack/swift,smerritt/swift,swiftstack/swift,smerritt/swift,smerritt/swift,matthewoliver/swift,nadeemsyed/swift,swiftstack/swift,openstack/swift,nadeemsyed/swift,openstack/swift,tipabu/swift,nadeemsyed/swift,matthewoliver/swift