Will the graduate school that I've been accepted to reconsider my admissions offer?
Almost certainly not. Every program/department/country is a little different, but in general I'd say a single lower-than-average grade at the last minute isn't a signal that someone isn't ready for graduate school. Especially in situations where the acceptance was based on far more than grades: statements of purpose, letters of recommendation, fit with the department, etc. If you failed a bunch of courses in your final semester, that might be seen as a red flag, but a single B- isn't likely to faze anyone in the slightest. You'd need a lot more than that to threaten your good standing (which in my experience is taken to mean 'not in any kind of trouble academically or otherwise').