Fixed incorrect highlighting of 'type'

The current code incorrectly highlights ‘type’ in

    Hol_datatype `sample = Foo | Bar`;

which is erroneous.

An example of this in the wild is
https://github.com/HOL-Theorem-Prover/HOL/blob/master/examples/machine-code/instruction-set-models/x86_64/x64_coretypesScript.sml