Fix errors in Blob comments

Spotted a few typos / inaccurate comments in the Blob class. Fix them.

Change-Id: I285a21307b2332bc2bdd84dce3fbb6728eeb8404
Reviewed-on: http://review.couchbase.org/41391
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>