[CONTRIBUTING]: Update Configlet Instructions for New Sync Command

[BethanyG]

After PR #366 in the Configlet repo is merged, we need to update the "Generating Practice Exercise Documents" section of our CONTRIBUTING file to reflect the new way to sync/generate Practice Exercise documents.

[J08K]

Hi @masterjacktexas,
I do not think this is the right place for promotion of Discord servers. Please refrain from promoting any further.
Thank you, and have a nice day.