update machine runner docs to use 3 instead of 3.0

[jrahme-cci]

Description

Reword Machine runner 3.0 to Machine runner 3 , as we've released a 3.1 minor version update earlier this year, but the instructions remain the same. Dropping the minor version will help avoid confusion.

Reasons

The docs looked to be out of date for an older version when they were not.

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).
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