Update index.rst

[lostdave]

Interval Syntax is wrong!

What does this PR do?

Corrects documentation

What issues does this PR fix or reference?

Fixes

Previous Behavior

Beacons would not start if - interval: XXX

New Behavior

Beacons start correctly

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• [ X] Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

No

Please review Salt's Contributing Guide for best practices, including the
PR Guidelines.

See GitHub's page on GPG signing for more information about signing commits with GPG.

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