fix(ngdocs): add ids to headers in tests

[mkolodny]

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.

[petebacondarwin]

Reverting to the previous version of marked (0.2.9) has removed the need for this change. Moving this to the post-1.2 milestone to be revisited when we decide to upgrade marked.

[IgorMinar]

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[Narretz]

ngdocs is no longer part of the doc system, closing