Changed \text to \operatorname to take care of nit.

The spacing seemed the same on this either way, but to be more proper I have changed it.

Test Plan:
Loaded http://localhost:8080/math/arithmetic/factors-multiples/greatest_common_divisor/e/greatest_common_divisor
and did a problem taking all the hints, making sure that it displays as before (but showing gcf not gcd)

Auditors: alpert
CC: eater