Correct Servo frequency to 50 Hz

[photodude]

Library incorrectly listed and used the update frequency for analog servos as 60 Hz, see references verifying analog servos should be run at 50 Hz updates

• https://dronebotworkshop.com/servo-motors-with-arduino/
  • "In a conventional analog servo motor a PWM signal with a period of 20 ms is used to control the motors. A signal of 20 ms has a frequency of 50 Hz."
• https://en.wikipedia.org/wiki/Servo_control#Pulse_duration
  • The period of 20 ms (50 Hz) comes from the days where the signal was encoded in PPM (pulse position modulation)
• Typical RC servo frequency is 50 Hz
  • see sg90 datasheet http://www.ee.ic.ac.uk/pcheung/teaching/DE1_EE/stores/sg90_datasheet.pdf

[photodude]

This change has been tested with our library extension which wraps this library, two Adafruit PCA9685 PWM Servo Drivers, an Arduino Uno, and 4 ESCs. The Internal oscillator frequency was adjusted using an oscilloscope until each Adafruit PCA9685 PWM Servo Driver produced a 50 Hz signal. With the correct 50Hz PWM signal the ESCs received the correct signal in their Servo receive line to calibrate, arm and drive the ESCs; prior to this change the ESCs behaved incorrectly and would not work in some cases. ESCs use the same 50Hz PWM signal as servos do.