That's it? She didn't show the lines of people waiting to get in to a book signing? He's attacking her not because she said something nasty about him, but because she didn't report the story in a way that he wanted the story reported, and in a way that would have promoted his campaign?
Yeah, THAT's the kind of guy I want to see as President. (/sarc)
No, IMO, he slammed her because she was up to the usual media tactic of slanting the news - in this case, showing tight shots of his book signing rather than a wide angle of the crowd. Had she shown that as well, instead of trying to minimize the size of the turnout, it would have been OK.
I'm sure you are aware of this tactic, which rightly came under fire.