Update repositories.txt (#4506)

Arduino library for the I2C LTR390 UV sensor.

Author: RobTillaart

repositories.txt

@@ -5088,6 +5088,7 @@ https://github.com/RobTillaart/LineFormatter
 https://github.com/RobTillaart/Logistic
 https://github.com/RobTillaart/LTC2991
 https://github.com/RobTillaart/LTR390_DFR
+https://github.com/RobTillaart/LTR390_RT
 https://github.com/RobTillaart/LUHN
 https://github.com/RobTillaart/M5ANGLE8
 https://github.com/RobTillaart/M5ROTATE8