docs: replace deprecated `GITHUB_COM_TOKEN` with `RENOVATE_GITHUB_COM_TOKEN`. While the former is still parsed by Renovate, it will remove it in a future major update. - Fixes #17

See https://docs.renovatebot.com/getting-started/private-packages/#looking-up-changelogs



Signed-off-by: kilianpaquier <noreply@kilianpaquier.dev>