nexthop: Fix usage output

nexthop: Fix usage output

Before:

 # ip nexthop help
 Usage: ip nexthop { list | flush } [ protocol ID ] SELECTOR
        ip nexthop { add | replace } id ID NH [ protocol ID ]
        ip nexthop { get| del } id ID
 SELECTOR := [ id ID ] [ dev DEV ] [ vrf NAME ] [ master DEV ]
             [ groups ] [ fdb ]
 NH := { blackhole | [ via ADDRESS ] [ dev DEV ] [ onlink ]
       [ encap ENCAPTYPE ENCAPHDR ] | group GROUP ] }
 GROUP := [ id[,weight]>/<id[,weight]>/... ]
 ENCAPTYPE := [ mpls ]
 ENCAPHDR := [ MPLSLABEL ]

After:

 # ip nexthop help
 Usage: ip nexthop { list | flush } [ protocol ID ] SELECTOR
        ip nexthop { add | replace } id ID NH [ protocol ID ]
        ip nexthop { get | del } id ID
 SELECTOR := [ id ID ] [ dev DEV ] [ vrf NAME ] [ master DEV ]
             [ groups ] [ fdb ]
 NH := { blackhole | [ via ADDRESS ] [ dev DEV ] [ onlink ]
         [ encap ENCAPTYPE ENCAPHDR ] | group GROUP [ fdb ] }
 GROUP := [ <id[,weight]>/<id[,weight]>/... ]
 ENCAPTYPE := [ mpls ]
 ENCAPHDR := [ MPLSLABEL ]

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>