When Marshall is used as someone's first or last name (like Marshall Tucker or the Marshall Plan) it is spelled with two "l's".
You’re right.
One time, my Aikido instructor was going to teach a
class at a community college, the class was advised
to wear loose clothing and be comfortable.
When they assembled, it was a nice day so He suggested
that they go out and practice on the grass.
Everyone was shocked.”You mean do it OUTSIDE?”.
Well, come to find that instead of Martial Arts
being printed on the flyers, someone had printed
“Marital Arts Class”. Hahahah.