docs: gator test --> gator verify (#1800)
docs: gator test --> gator verify (#1800)

* Update docs: gator test --> gator verify

A previous PR (#1799) changed the gator test command to now be `gator
verify`.  That PR left out docs changes.  Those docs updates are
included here.

Signed-off-by: juliankatz <dd17a9ef9fe3f98508483843602a7a598434a193@google.com>
open-policy-agent/gatekeeper,open-policy-agent/gatekeeper,open-policy-agent/gatekeeper,open-policy-agent/gatekeeper,open-policy-agent/gatekeeper