another XSweet math fix
XSweet sometimes sends back math in the form:
<h3></h3><h3><math-display>[math]</math-display></h3>
This doesn't work and results in <h3></h3><h3>[math]</h3>
because <math-display>
is a block-level tag and Wax says it can't exist inside of a header. (What should be coming back is straight <math-display>[math]</math-display>
which a pending XSweet microservice MR should fix.) This pulls <math-display>
out of headers – previously we were just pulling it out of paragraphs.