Update /manual/configure-github-repo.html.md
Update /manual/configure-github-repo.html.md

This fixes a broken link to a Jenkins job and refers to `main` as the
default branch, relegating `master` as legacy option in parenthesis.

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs