nfsidmap: Manpage corrections

I recently upgraded to 1.2.4 to use the new nfsidmap feature. While
following the manpage and the kernel documentation I noticed a
difference. Kernel docs mention key TYPE id_resolver while the manpage
states nfs_idmap. The following patch changes the manpage to the proper
type.

Signed-off-by: Steve Dickson <steved@redhat.com>