dm cache: Documentation: update default migration_throttling value

In commit f8350daf7af0 ("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>
diff --git a/Documentation/device-mapper/cache.txt b/Documentation/device-mapper/cache.txt
index 79c7b6d..ff08417 100644
--- a/Documentation/device-mapper/cache.txt
+++ b/Documentation/device-mapper/cache.txt
@@ -119,7 +119,7 @@
 
 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
 -------------------------