Reformats markdown problem templates avoid extra line breaks.

Based on the parsing rules in `common/lib/xmodule/xmodule/js/src/problem/edit.coffee` every line is formatted as a new paragraph (line 400):

    splits[i] = splits[i].replace(/(^(?!\s*\<|$).*$)/gm, '<p>$1</p>');

I've simply reformatted the template files so they are consistent with their generated xml in order to avoid changing existing behavior.

@cahrens @andy-armstrong