docs/security/sheriff.md: Change the wording for V8 issues severity guidance.

This change should've been uploaded as https://crrev.com/c/1623505, but
somehow uploading failed on my end and I haven't noticed it back then.

TBR=adetaylor@chromium.org

Change-Id: I75eae549d6a8976e9ad8dcd550e065b22bf7211a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626334
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Commit-Queue: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663041}
1 file changed