Ubuntu setup - clarify step to check rbenv version

Step 5 of the Ubuntu setup instructions include a check for `rbenv version` inside the description and then suggestions on what to do if that does not produce expected output. I'd recommend moving the `rbenv version` step to a code block to make it easier to follow.

Start with Step 5 of the Ubuntu setup instructions and locate the inline `rbenv version` check. Move that command into its own code block while preserving the surrounding troubleshooting guidance; done means the step is easier to follow and the expected-output instructions remain clear.

Written by the indexing model from the issue text.