It's just the way laughter in many Germanic languages is written. Whereas you are I write/type "Hehehehehe", those who speka Germanic languages such as German, Danish, Norsh, Swedish etc, would type it as "Hihihihihihi". Bewitched is German, so she types it "Hihihihihihi".

It's got something to do with the sounds the "i" makes in said languages. In English, it's more of an "eye" sound, whereas in German it's an "ee" sound.