Update readme so it specifies correct ruby version

What does this MR do and why?

Hi 👋 This is the first MR of mine to this repo (and on GitLab even), so let me know if I seem to miss something 🙂

I've found a minor discrepancy of ruby versions between .ruby-version and README.md, where changing the latter seems to have been forgotten in !105167 (merged). So I edited the README.md.

If you think you feel more comfortable to keep it only in .ruby-version and off README.md I believe that'd be a good one too. Here, however, I focused on just arranging them right and kept from arguing the preference.

Screenshots or screen recordings

I'm sure this is alright without them.

How to set up and validate locally

I think this MR is not applicable.

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