backpressure_disk_limiter: fix comment
backpressure_disk_limiter: fix comment

Suggested by songgao.

Issue: #1033

keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs