(#19476) Correct source path for terminus code in docs

The terminus code in the documentation was referenced using an incorrect path.

This patch makes sure we reference the correct path for installation from
source.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #480 from kbarber/ticket/master/19476-install_from_source_path

(#19476) Correct source path for terminus code in docs