The "H" or High limit screw controls how far the derailleur can travel towards the outside. This is the 1 you might want to micro-adjust., by uncrewing (counter-clockwise) a 1/4 turn or so (as others mentioned already)
The "L" Low limit screw controls how far into the spokes you want the derailleur to go.
You can get some info here too:
Park Tool Co. » ParkTool Blog » Rear Derailler Adjustments (derailleur)