Make it clear that allowlisting is for third-party Snaps

[Montoya]

Description

Slight modification and clarification to "Get Allowlisted" page for Snaps.

Issue(s) fixed

n/a

Preview

https://docs.metamask.io/allowlisting-audit-caveat/snaps/how-to/get-allowlisted/

Checklist

Complete this checklist before merging your PR:

• [ ] If this PR contains a major change to the documentation content, I have added an entry to the top of the "What's new?" page.
• [ ] The proposed changes have been reviewed and approved by a member of the documentation team.

[github-actions[bot]]

Preview published: allowlisting-audit-caveat