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: NJohn Pittman <jpittman@redhat.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>

Documentation/device-mapper/cache.txt

@@ -119,7 +119,7 @@ doing here to avoid migrating during those peak io moments.
 
 For the time being, a message "migration_threshold <#sectors>"
 can be used to set the maximum number of sectors being migrated,
-the default being 204800 sectors (100MB).
+the default being 2048 sectors (1MB).
 
 Updating on-disk metadata
 -------------------------