Actually, “step out” would work better in that sentence. To “step out” means to go away (or go out) from a place for a short period of time. “I am stepping out for a little bit.” “A step away” is a very short distance (i.e. the distance covered by a step). You could use “step away” to mean “move away” or “take a step away” from something (e.g. “Step away from the gun!”). If the sentence was “I am stepping away a little bit” (without the “for”) then it would mean you were taking a few steps away from where you were.