fix(ngdocs): add ids to headers in tests

Doc.markdown (in /docs/src/ngdoc.js) gives headers ids, yet headers don't have ids in the specs.
That's causing the tests "ngdoc markdown should replace text between two [pre] tags", and "ngdoc TAG @description should support multiple pre blocks" to fail when building the documentation.
This change adds ids to headers in the specs to match the behavior of Doc.markdown.