doc: osd_backfill_scan_(min|max) are object counts

osd_backfill_scan_min and osd_backfill_scan_max set the number of
items grabbed during a single backfill scan, not an interval in
seconds. Correct the doc.

Signed-off-by: Dan van der Ster <daniel.vanderster@cern.ch>
Merge pull request #2488 from cernceph/docfix

doc: osd_backfill_scan_(min|max) are object counts

Reviewed-by: Loic Dachary <loic-201408@dachary.org>