Depending on how kinda broke the stem is, you could use some garden twist ties to support the branch until it heals. However, so close to harvest you may be better off drying, quick curing, and enjoying some bud nouveau. I've grown to welcome a broken bud or two near harvest, just so I could get an early sample. Indeed, just had it happen to a 1g (dried) bud frown from some home-bred White Widow Extreme seed that had started to mature. It broke off a couple of weeks ago, dried it and stuck it in a Boveda humidity-controlled bottle with some other cured bud for a week. First sample! Good lemon taste up front with a peppery finish and good, well-rounded high.
Relax. Enjoy. These are the kind of mistakes we can smoke.