Change mirror disk requirements documentation

[aminvakil]

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)

[sspencerwire]

Good catch @aminvakil I'll merge this change. It looks like the document could use some other love too. Thanks for the PR!