use non-greedy *? for display math content

[kleinschmidt]

Using greedy * means that the regex will match across display math blocks. For example, the current regex matches this whole snippet (including the non-math bit in the middle):

\[
some math
\]
non-math
\[
more math
\]

The only potential downside I can see is that if, for some strange reason, someone wants a literal \] inside their display math block, the regex will think that the block ends early. But this seems less likely than the frustrating situation of having non-math font-locked as math in between two display math blocks (which is very common in my documents).

[kleinschmidt]

I've opened this at jrblevin/markdown-mode#168