Yup, the cable has simply stretched and the derailleur is no longer lining up with the gear properly. Just twist that little black knob where the cable goes into the shifter until it lines up with the gear you want it to be in. I find it easiest to set it to the middle gear and adjust for that.
If you've got someone to hold the bike up, they can hold the bike while you twist and then spin the pedal and try shifting, it'll let you get it dialed in really fast.