Update installation.md

[alemorvan]

There is an extra space before the underscore

Author checklist (to be completed by original Author)

• Is this document a good fit for the Rocky project ?
• Is this a non-English contribution?
• Title and Author MetaTags have been inserted into the document
• If applicable, steps and instructions have been tested to work on a real system
• Did you perform an initial self-review to fix basic typos and grammatical correctness

Rocky Documentation checklist (to be completed by Rocky team)

• 1st Pass (Check that document is good fit for project and author checklist completed)
• 2nd Pass (Technical Review - check for technical correctness)
• 3rd Pass (Basic Editorial Review)
• 4th Pass (Detailed Editorial Review and Peer Review)
• 5th Pass (Include document in TOC)
• Final pass/approval (Final Review)