dm cache: Documentation: update default migration_throttling value

In commit f8350daf ("dm cache: tune migration throttling") the
value for DEFAULT_MIGRATION_THRESHOLD was decreased from 204800 to
2048.  Edit device-mapper/cache.txt to reflect the correct default
value for migration_threshold.

Signed-off-by: John Pittman <jpittman@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>