docs: Fix wording in analog IO PWM figure reference.

Author: tdicola

docs/programming_guide/02_analog_io.rst

@@ -429,7 +429,7 @@ To further illustrate how PWM is different from true analog output, look at the
 image below which shows oscilloscope output of a PWM signal at different duty
 cycles (0%, 25%, 50%, 75%, and 100%).  Notice how as the duty cycle increases
 the amount of time the signal is at a high logic level (3.3V) gets longer.  At
-75% duty cycle the signal is high for twice as long as at 25% duty cycle
+50% duty cycle the signal is high for twice as long as at 25% duty cycle
 (compare how long the tops of each wave are to check for yourself).  At the
 extremes of 0% and 100% you can also see the signal never changes and is always
 at a high or low level!