docs: rework coverage regex examples to work around a markdown escaping issue with the nyc example

What does this MR do?

Code coverage regex example for nyc (NodeJS) rendered badly from markdown in self-hosted GitLab help and in the repo preview as well (see https://gitlab.com/gitlab-org/gitlab/-/blob/3dd4985454e2308ac3aa77444e46ffbd0bfe5e16/doc/ci/pipelines/settings.md). Online html docs on the opposite shows the regexp correctly.

This is due to missing escape for | character within the table which this MR attempt to fix. Moreover literal \ should be escaped as well to show up.

Related issues

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