[MPL/doc] Fix naming of the documentation file
Related to issue posted in the forum: https://discourse.opengeosys.org/t/error-the-property-type-relative-permeability-nonwetting-vangenuchten-was-not-recognized/1005
Related to issue posted in the forum: https://discourse.opengeosys.org/t/error-the-property-type-relative-permeability-nonwetting-vangenuchten-was-not-recognized/1005