How can I correct a crowned floor joist?
Pretty simple to fix if you have access from underneath. I would start by adding a couple of 2x4 with screws underneath several of the joists on each side of the problem joist. 

Next, cut the joist vertically at the location where the bump is. This will let the joist sag down on top of the 2x4 braces that you added. 

Finally add a sister next to the joist that you cut. The simplest thing to do is to use a piece of wood of equivalent size as the existing joist. It is glued onto the existing joist with construction adhesive and then bolted together securely with closely-spaced bolts.