Added "Heatit ZM Single Relay" in the table in things.md with link to existing markdown documentation.
Also corrected a typo and wrong category description.
Note that the database will overwrite the files relating to the device, so I'll merge this, but it will then be changed back in a few days unless the database is updated.
Added "Heatit ZM Single Relay" in the table in things.md with link to existing markdown documentation. Also corrected a typo and wrong category description.