[SafeBrowsing] Clarify `danger_level` in safe_browsing/README.md

BUG=none
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2076873003
Cr-Commit-Position: refs/heads/master@{#401036}
1 file changed