forth: Finish closing #961 - update example (#1033)
forth: Finish closing #961 - update example (#1033)

As discussed in #1032, the PR was merged too quickly, so this is to
complete the contribution by updating the exercise's example in order to
be compatible with the modified unit tests.

Signed-off-by: Paul Mabileau <a3aedf8c87c1af3dfeae5f57ddf6d5ae82ad981f@hotmail.fr>
petertseng/exercism-rust,petertseng/exercism-rust,exercism/xrust,petertseng/exercism-rust,exercism/xrust