Update repositories.txt (#5809)

Add millis64, an Arduino library for millis64 micros64 millis32 micros32, time counters with optional offset.

Author: RobTillaart

repositories.txt

@@ -5493,6 +5493,7 @@ https://github.com/RobTillaart/MCP4261
 https://github.com/RobTillaart/MCP4725
 https://github.com/RobTillaart/MCP9808_RT
 https://github.com/RobTillaart/MHZCO2
+https://github.com/RobTillaart/millis64
 https://github.com/RobTillaart/MiniMP3
 https://github.com/RobTillaart/MINMAX
 https://github.com/RobTillaart/ML8511