Fix swapped max and default diff settings in mining post

Summary:
I didn't test this myself, but the example config at https://github.com/viabtc/viabtc_mining_server/blob/main/gateway/config.json#L51 indicates these numbers are backwards.
Just reading it seems obvious enough to write this patch.

Test Plan:

bundle exec jekyll serve

Check http://127.0.0.1:4000/2021-07-06-mining-xec-using-viabtc-software/

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11399