kbfs_ops_concur_test: reword test comment
kbfs_ops_concur_test: reword test comment

Suggested by jzila.

Issue: #663

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