Removes reference to using multiple machine runners on one instance

[JoeMitchellJones]

Removes an outdated line stating there can be multiple runners on one machine. This is no longer supported on machine runner 3

Description

Removes a sentence presumably taken from the launch agent docs that refers to multiple runners on one host.
Redo of #9124

Reasons

Running multiple machine runner 3 instances on one host is not supported, so this was giving false information.

Content Checklist

Please follow our style when contributing to CircleCI docs. Our style guide is here: https://circleci.com/docs/style/style-guide-overview.

Please take a moment to check through the following items when submitting your PR (this is just a guide so will not be relevant for all PRs) 😸:

• Break up walls of text by adding paragraph breaks.
• Consider if the content could benefit from more structure, such as lists or tables, to make it easier to consume.
• Keep the title between 20 and 70 characters.
• Consider whether the content would benefit from more subsections (h2-h6 headings) to make it easier to consume.
• Check all headings h1-h6 are in sentence case (only first letter is capitalized).
• Is there a "Next steps" section at the end of the page giving the reader a clear path to what to read next?
• Include relevant backlinks to other CircleCI docs/pages.