Travis runs check-links.js to make sure links in *.md files are not dead. For some reason, a dead link was not caught by check-links.js

pull request: #26539
dead link: https://github.com/ampproject/amphtml/master/extensions/amp-consent/cmps/appconsent.md
Travis job that should have failed but didn't: https://travis-ci.org/ampproject/amphtml/jobs/645341567#L418