Dual variable power supply with LM337 and LM317
The simple solution is to install a 4.7K in series with each of the 5K pots leads going to ground. That should cut the 30vdc to about 15 vdc for both positive and negative supplies, at the 'maximum' setting. That is two 4.7K resistors total.

Since the LM317/LM337 series have an internal 1.25vdc reference, the easy fix is to install series diodes in the outputs, after the capacitors, causing a 1.25 volt drop. Two each 1N5408 3 amp diodes plus using 2.2K 1 Watt resistors across the outputs as minimum loads will bring the outputs close to zero volts at the 'zero' setting. That is 4 1N5408 diodes total and 2 2.2K 1Watt resistors total.

Be sure to heatsink the IC's if drawing more than 200mA from them. Also, a room temperature change or regulator change of 20 degrees F will cause at least a 100mV change in the output voltage. These are general purpose voltage regulators, not precision regulators.

There are more accurate solutions that use zener diode and op-amps and trim-pots, but I was not sure what level of priority that had.

EDIT: Your probably using 5% resistors, so don't expect precise tracking or that maximum volts is = +15.00, etc. Or that zero volts is equal to 0.00. Or that a substantial change in temperature of U1 or U2 causes a change in voltage. If you want better accuracy replace the 4.7K resistors with 10K trim pots. Put their wiper positions in the center to start with, then trim for max voltage = +/-15.00vdc.