Auto merge of #4414 - chansuke:fix-contributing-doc, r=phansch
Auto merge of #4414 - chansuke:fix-contributing-doc, r=phansch

Fix the name of a channel of discord in CONTRIBUTING.md

Fixes #4400.

changelog: none

Update the naming of the Discord channel.

graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust