Scaling level of PWM signals
You can simplify the circuit by using a Broadcom ACPL-021L logic-output optoisolator.
The input resistor must be increased, something like 7.5K would work. Just wire the output side to +5/GND and the servo input and add a bypass capacitor.

That particular part is non-inverting, like your circuit (input high = output high). If you can use inverting, then there are other more common options in logic-output optoisolators.
The input signal at the servo input should be a positive 3-5V pulse of width 1.0 to 2.0 milliseconds (1.5ms representing a position at about half travel), repeated at some tens of Hz frame rate (non-critical).