prevent extra p surrounding multiple choice ul in PTX

[Alex-Jordan]

This should have been included in #1090.

Without getting into the weeds too much, consider something like:

$rational = DropDown(["is","is not",], 1);

BEGIN_PGML
The number [`\sqrt{2}`] [_]{$rational}{5} rational.

Without this commit, the PTX output is like:

<p>The number <m>\sqrt{2}</m> <p><ul form="popup" ...> ...

Note the p within a p.

With this commit, the PTX output is:

<p>The number <m>\sqrt{2}</m> <ul form="popup" ...> ...

[somiaj]

I can see the additional regex matches your description, but again don't have a way to test.

[drgrice1]

Looks good.