Update repositories.txt (#6719)

https://github.com/adafruit/Adafruit_QMC5883P

Author: ladyada

repositories.txt

@@ -744,6 +744,7 @@ https://github.com/adafruit/Adafruit_Pixie
 https://github.com/adafruit/Adafruit_PM25AQI
 https://github.com/adafruit/Adafruit_Protomatter
 https://github.com/adafruit/Adafruit_PyCamera
+https://github.com/adafruit/Adafruit_QMC5883P
 https://github.com/adafruit/Adafruit_RA8875
 https://github.com/adafruit/Adafruit_S-35710
 https://github.com/adafruit/Adafruit_SCD30