CONTRIBUTING.md: Update rustfmt documentation

3 jobs from alatiera/rustfmt-docs in 8 minutes and 6 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #104601
fedora test

00:06:29

 
  Lint
manual #104603
allowed to fail manual
clippy
passed #104602
rustfmt

00:01:36