Update repositories.txt (#4378)

Adding Sensirion libraries for SHT3x, STS3x and SFM-SF06 sensors

Author: LeonieFierz

repositories.txt

@@ -6869,3 +6869,6 @@ https://github.com/Embeddronics-ltd/MS5837_30BA_Library
 https://github.com/HighASG936/hw262
 https://github.com/gershnik/MbedNanoTLS
 https://github.com/gershnik/BetterWiFiNINA
+https://github.com/Sensirion/arduino-i2c-sfm-sf06
+https://github.com/Sensirion/arduino-i2c-sht3x
+https://github.com/Sensirion/arduino-i2c-sts3x