tc: add a missing space between rate estimator and backlog

tc: add a missing space between rate estimator and backlog

When a rate estimator is active, "tc -s qd" displays
something like :

rate 12616bit 11ppsbacklog 0b 0p requeues 2

instead of :

rate 12616bit 11pps backlog 0b 0p requeues 2

Fixes: 4fcec7f3665b ("tc: jsonify stats2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>