Update repositories.txt (#6182)

Arduino library for the PPD71 Particle Sensor Unit.

Author: RobTillaart

repositories.txt

@@ -5645,6 +5645,7 @@ https://github.com/RobTillaart/PinInGroup
 https://github.com/RobTillaart/PinOutGroup
 https://github.com/RobTillaart/PIR
 https://github.com/RobTillaart/PIR8575
+https://github.com/RobTillaart/PPD71
 https://github.com/RobTillaart/Prandom
 https://github.com/RobTillaart/pressure
 https://github.com/RobTillaart/PrintCharArray