Update proof count on index.html

[CatsAreFluffy]

https://us.metamath.org/ currently claims that there are "over 23,000 proofs" in the Metamath Proof Explorer. While technically true, there are in fact currently over 41,000 proofs in set.mm. This should be updated.

[jkingdon]

I think you just need to make a pull request for https://github.com/metamath/metamath-website-seed/blob/main/index.html