This commit addresses exactly two issues at once:
1. The HTG unit tests exercising the case where a 0-level material
mask array is passed (most notably in order to allow for large
meshes where only a relatively small number of root cells are
concerned by a particular material) where using a confusing (yet
useless) underscore marker in their descriptions, which resulted
in confusion from some users. These have been removed.

2. An embarrassing spelling mistake, thrice repeated, was fixed.