Pearl;

You need to give more information...did it break off while trying to loosen it in preperation for bleeding (from being rusted in place, because it didn't have anti-seize applied on the threads during installation), condition 1; or did it break from overtightening while you doing the bleeding (it's not a headbolt, and maybe you should consider having someone else do your mechanicking, if you don't know your own strength!), condition 2.

Repairs suggestions:

Condition 1. Replace wheel cylinder, removing the remains of the bleeding nipple while on the vehicle is just about impossible.

Condition 2. Use an easy-out type of extractor to remove the remains (not fun while on the vehicle either), then replace bleeding nipple (they are available individually or used), not using 1/2" drive sockets to tighten...

Use anti-seize on every thread in sight...

Someone may have a used master cylinder cap for you too...

Good Luck