Update repositories.txt (#5717)

Add library for AGS3871 Carbon Monoxide sensor

Author: RobTillaart

repositories.txt

@@ -5336,6 +5336,7 @@ https://github.com/RobTillaart/Adler
 https://github.com/RobTillaart/ADS1X15
 https://github.com/RobTillaart/ADT7470
 https://github.com/RobTillaart/AGS02MA
+https://github.com/RobTillaart/AGS3871
 https://github.com/RobTillaart/AM2315
 https://github.com/RobTillaart/AM2315C
 https://github.com/RobTillaart/AM232X