Update installing_ruby.md

[jojodm997]

Because

Linux system update commands don’t need to be on two separate lines.

This PR

Updated the Linux system update command.

Issue

Closes #XXXXX

Additional Information

Pull Request Requirements

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• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
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[KevinMulhern]

Thanks for the PR @jojodm997, but most of our users will be new to linux, and for those learners specifically, I think there is value with making it obvious these are two separate commands.