[doc] Simplify example for building the Verilator model
[doc] Simplify example for building the Verilator model

This is simpler and has the benefit of matching what we do in CI (so
we know it will continue to work).

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan