Correct "copier" docs

In https://guidebook.devops.uis.cam.ac.uk/en/latest/howtos/copier-migrate/, the _src_path in the copier config should read git+https://gitlab.developers.cam.ac.uk/uis/devops/webapp-boilerplate.git and git+https://gitlab.developers.cam.ac.uk/uis/devops/gcp-deploy-boilerplate.git.