Update repositories.txt (#6885)

Arduino library for AS7331 UV sensor

Author: RobTillaart

repositories.txt

@@ -5708,6 +5708,7 @@ https://github.com/RobTillaart/AngleConvertor
 https://github.com/RobTillaart/ANSI
 https://github.com/RobTillaart/APDS9900
 https://github.com/RobTillaart/AS5600
+https://github.com/RobTillaart/AS7331
 https://github.com/RobTillaart/AsyncAnalog
 https://github.com/RobTillaart/AtomicWeight
 https://github.com/RobTillaart/AverageAngle