[llvm] [doc] Improve wording for "What is considered a security issue?" (PR #69436)

[Peter Smith via llvm-commits]

https://github.com/smithp35 approved this pull request.

LGTM, was in line with what we discussed in the last security meeting, one word that looks like a typo, although this was already there.

I've marked approved on my side, will be good to hear from the other group members too.

https://github.com/llvm/llvm-project/pull/69436