gnt-instance rename: Update the man-page
gnt-instance rename: Update the man-page

In commit 31fe5102a a additional check was added. This patch rephrases
the man page accordingly to address this change.

Signed-off-by: René Nussbaumer <bc755ac45a0e799d1d490cba76febc56d80cc5fc@google.com>
Reviewed-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>

andir/ganeti,yiannist/ganeti,dimara/ganeti,leshchevds/ganeti,leshchevds/ganeti,ganeti/ganeti,leshchevds/ganeti,onponomarev/ganeti,ganeti-github-testing/ganeti-test-1,mbakke/ganeti,andir/ganeti,bitemyapp/ganeti,grnet/snf-ganeti,andir/ganeti,grnet/snf-ganeti,bitemyapp/ganeti,apyrgio/ganeti,onponomarev/ganeti,apyrgio/snf-ganeti,yiannist/ganeti,dimara/ganeti,mbakke/ganeti,ganeti/ganeti,ganeti/ganeti,yiannist/ganeti,mbakke/ganeti,apyrgio/snf-ganeti,apyrgio/ganeti,ganeti-github-testing/ganeti-test-1