add one word to threat.yml

[djptek]

• Have you signed the contributor license agreement? Y
• Have you followed the contributor guidelines? Y
• For proposing substantial changes or additions to the schema, have you reviewed the [RFC process] (https://github.com/elastic/ecs/blob/master/rfcs/README.md)? Y
• If submitting code/script changes, have you verified all tests pass locally using make test? Y
• If submitting schema/fields updates, have you generated new artifacts by running make and committed those changes? Y
• Is your pull request against master? Unless there is a good reason otherwise, we prefer pull requests against master and will backport as needed. Y
• Have you added an entry to the CHANGELOG.next.md? Y

This is a one word addition, improves readability in docs.

[kgeller]

Don't forget to add in the PR number!

[djptek]

Oops! Added, Thanks @kgeller

[kgeller]

LGTM after the changelog addition