This mod takes the tone control from the middle pickup and puts it to use for the bridge pickup. Just follow the schematic below. Note that the lime green wire can simply be unsoldered from the lug on the pickup selector and resoldered at the new location to take on the configuration described by the black wire deviating from the original green. (Just ignore the blue wire passing through the neighborhood.)
In this diagram, the gray wires go to ground. Other colors were chosen arbitrarily.
Wherever you see a ground symbol next to a potentiometer, the wire should be grounded to the metal on the back of the pot. After that, all of the grounds should all go to one point to avoid ground loops.
For the sake of simplicity, the string ground is not pictured in this diagram (but you absolutely need to connect it!). See the "All About Grounding" page to learn more.
Note: The image above is larger than displayed. If you wish to print it, save it and resize as desired to take advantage of the actual resolution.
I you want to use the tone control to modify both the middle and bridge pickups, just add the new black wire to the green (or just run between the two lugs on the pickup selector switch), rather than moving the green wire to the new connection as described above.
In the unlikely event you feel you would like to have both options, you can also use any SPDT switch (a standard DPDT push-pull will work too) to switch between these configurations. Just connect the tone pot to the middle ("common") lug on the switch, and solder the appropriate lugs on the pickup selector switch to the respective lugs on either side of the common. You decide which is appropriate for you.