dependabot.yml: header comment claims "Two ecosystems" but actually tracks three (npm/actions/docker)

[CryptoJones]

Problem

.github/dependabot.yml opens with:

# Two ecosystems get watched:
#   1. npm — the production + dev tree in package.json.
#   2. github-actions — workflow action versions in .github/workflows/.
#   3. docker — base images in Dockerfile / docker-compose.yml.

…then lists three. The docker ecosystem block was added in a follow-up commit and the leading count never got updated. Pure documentation rot.

Fix

Replace Two with Three. One-character change.

Proudly Made in Nebraska. Go Big Red! 🌽 https://xkcd.com/2347/