The Free Dictionary gives these alternatives for knob (sense 2):

• A rounded handle, as on a drawer or on a door.

• A rounded control switch or dial.

Both work. What is the handle, the little bit you turn to get the water flowing?

Merriam-Webster has a poor definition, in my opinion, because the definition of “a small rounded ornament or handle” isn’t really broad enough to cover the wide variety of controls that I would call knobs. In my dialect “knob” may be contrasted with “button”: a knob is a control you turn, a button is a control you push.