Fix markdown rendering with inline maths

The mistune 'text' regex was capturing into math expressions in some
cases. The fix was to modify it to stop at a $ sign, giving the inline
math regex a chance to capture that.

Closes gh-6724

IPython/nbconvert/filters/markdown.py

@@ -96,6 +96,7 b' class MathBlockLexer(mistune.BlockLexer):'
 @undoc
 class MathInlineGrammar(mistune.InlineGrammar):
     math = re.compile("^\$(.+?)\$")
+    text = re.compile(r'^[\s\S]+?(?=[\\<!\[_*`~$]|https?://| {2,}\n|$)')
 
 @undoc
 class MathInlineLexer(mistune.InlineLexer):