Update repositories.txt (arduino#5474)

Author: randyb128

repositories.txt

@@ -7535,5 +7535,6 @@ https://github.com/styropyr0/oled.h
 https://github.com/jaakka/DelayFunctionsArduino
 https://github.com/ZanPekosak/SlowPWM
 https://github.com/Lorandil/uCompression
+https://github.com/DMT-Services/GeoMagnetism
 https://github.com/ConsentiumIoT/ConsentiumStarterKit
 https://github.com/batuhantrkgl/ESP8266-D1_SMTPClient