Table clarifying access to course material missing setting read-open-time

The table in the section [Student's access to course material over time](https://plus.cs.aalto.fi/aplus-manual/master/admin/settings/#student-s-access-to-course-material-over-time) on the page _Course Settings_ is missing mention of **read-open-time**, which is documented in the [a-plus-rst-tools README -file](https://github.com/apluslms/a-plus-rst-tools#6-meta-exercise-round-settings).