Grades in masters before PhD
First of all: Don't Panic!

If you search for Bad Grade on this site you get a lot of questions from students in a similar/worse situation. In this case I can just talk for me and for my case and let the other questions talk for themselves: I'm currently a PhD-student and I've gotten among others one B, one C and even one D in courses during my masters. In the end nobody cared because I've written an really good thesis and (at least here at my university and group) that's way more important. So focus on what's ahead of you.

So all in all it is of no significance (or at least should not be).