CONTRIBUTING.md: Use a generic reference name for the issues link

Downstream lesson maintainers are supposed to adapt this to point at
their repository's issues [1], so use a reference name that works for
both the lesson-template and downstream repositories.  That way they
only have to update the referenced URL and can leave the reference
name alone.

[1]: https://github.com/swcarpentry/lesson-template/issues/195#issuecomment-78310923