Update solids properties base names

https://mfix.netl.doe.gov/gitlab/exa/mfix/-/merge_requests/1820 changes how molecular weights are defined

• fluid docs already have [fluid_name] as the base for molecular weight // no change needed
• solids docs list base as solids. This MR updates the be base to [solid_name]

https://mfix.netl.doe.gov/gitlab/exa/mfix/-/merge_requests/1798 changes how molecular viscosity is defined

• fluid docs already have [fluid_name] as the base for molecular viscosity // no change needed
• solids do not take molecular viscosity as an input // no change needed

https://mfix.netl.doe.gov/gitlab/exa/mfix/-/merge_requests/1791 changes how specific heat inputs are defined

• fluid docs already have [fluid_name] as the base for specific heat // no change needed
• solids docs list base as solids. This MR updates the base to [solid_name]