The html has
href="../../reference/obook-maturity-indicator/"
I guess it should be
href="../../../reference/obook-maturity-indicator/"
(or change to an absolute URL, but hat also has disadvantages)
Due to the problem that you can never know for sure which level in the page you are, I updated the link (and the example snippet) to use an absolute URL..
For example, if you click the maturity indicator on https://oboacademy.github.io/obook/lesson/contributing-to-obo-ontologies/, it goes to https://oboacademy.github.io/obook/lesson/reference/obook-maturity-indicator but should go to https://oboacademy.github.io/obook/reference/obook-maturity-indicator/
The html has href="../../reference/obook-maturity-indicator/" I guess it should be href="../../../reference/obook-maturity-indicator/" (or change to an absolute URL, but hat also has disadvantages)