Correct documented commands for checking failed background migrations

What does this MR do and why?

Corrects the documented commands for checking failed background migrations so that they display/print the actual count.

https://docs.gitlab.com/ee/update/index.html#checking-for-background-migrations-before-upgrading

Screenshots or screen recordings

N/A.

How to set up and validate locally

N/A (simple documentation change).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.