Add note about angle brackets to Dropdown topic

@lamagnifica @explorerleslie Please review.
Thanks for bringing this up. "Common problems" is left over from when exercise types were arranged according to the Studio UI. Rather than saying "common problems," it would make more sense to change the text to something like "problems in the Simple Editor." This text occurs in several different problem types, however, so this change is out of the scope of this PR. I'll add a comment to DOC-950.