Adjust language in global security advisories intro

[bdragon]

Why:

Minor copy edit.

What's being changed (if available, include any code snippets, screenshots, or gifs):

I believe Global security database live in the GitHub Advisory Database was an error. Additionally [...] advisories live in the [database], which contains [...] advisories felt redundant so I adjusted the language slightly.

Check off the following:

• [ ] I have reviewed my changes in staging (look for the "Automatically generated comment" and click the links in the "Preview" column to view your latest changes).
• [ ] For content changes, I have completed the self-review checklist.

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This comment is automatically generated and will be overwritten every time changes are committed to this branch.

The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
code-security/security-advisories/global-security-advisories/about-global-security-advisories.md	fpt ghec ghes@ 3.7 3.6 3.5 3.4 3.3 ghae	fpt ghec ghes@ 3.7 3.6 3.5 3.4 3.3 ghae

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fpt: Free, Pro, Team ghec: GitHub Enterprise Cloud ghes: GitHub Enterprise Server ghae: GitHub AE

[cmwilson21]

👋 @bdragon Thanks so much for opening a PR! I'll get this triaged for review :zap:

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