Fix active grandchild link class (#962)

An occurrence of `grandchild` was miss-spelled `grand_child` (!).
That caused omission of the `active` class in the link in the nav panel for all grandchildren.

This bug was discovered using `diff` to compare the generated sites built using v0.4.0.rc1 and commit  4396b6b1c836f354bb7aa7edc1a06a49f137d615 on the fix-nav-performance PR branch.