Bump govuk-markdown from 0.7.0 to 0.8.0 (#743)

Bumps [govuk-markdown](https://github.com/x-govuk/govuk-markdown) from 0.7.0 to 0.8.0.
- [Release notes](https://github.com/x-govuk/govuk-markdown/releases)
- [Commits](x-govuk/govuk-markdown@v0.7.0...v0.8.0)

---
updated-dependencies:
- dependency-name: govuk-markdown
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

package.json

@@ -33,7 +33,7 @@
     "express-writer": "0.0.4",
     "fancy-log": "^2.0.0",
     "govuk-frontend": "^5.5.0",
-    "govuk-markdown": "^0.7.0",
+    "govuk-markdown": "^0.8.0",
     "gulp": "^5.0.0",
     "gulp-nodemon": "^2.5.0",
     "gulp-sass": "^5.0.0",