MRP-1998 ptlrpc: grammar fix.

ptlrpc_invalidate_import() error message grammar fix.

Change-Id: Ic1a99440f381ed982e348267996e4523aef8ebad
Signed-off-by: Alexander Zarochentsev <alexander.zarochentsev@seagate.com>
Reviewed-on: http://es-gerrit.xyus.xyratex.com:8080/4203
Tested-by: Jenkins
Tested-by: Elena Gryaznova <elena.gryaznova@seagate.com>
Reviewed-by: Colin Faber <colin.faber@seagate.com>
Reviewed-by: Vitaly Fertman <vitaly.fertman@seagate.com>