The rock will go faster because more elastic energy was built into the system by stretching it further. Elastic energy is a type of potential energy that is stored by bending or stretching something out of it's normal configuration, or shape. When the object being bent or stretched is released, it immediately springs back to it's original shape or configuration, converting the stored elastic energy into another type of energy, kinetic energy. Kinetic energy is the energy of motion, or the energy of things that are moving. The more you stretch the elastic of the slingshot, the more energy you are storing into place to be converted upon release into energy of motion. That is why the rock goes faster if you stretch the elastic farther.
Potential energy :- change in position of an object leads to posses potential energy, here when you pull the string you changed the position of the string and gave it potential energy, when you leavd the string the potential energy gets converted into kinetic energy and led it to move.