Update repositories.txt

[RobTillaart]

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

[github-actions]

Arduino Lint has suggestions for possible improvements to https://github.com/RobTillaart/millis64:

Linting library in millis64
WARNING: Invalid category field value Processing in library.properties                                         
         See: https://arduino.github.io/arduino-cli/latest/library-specification/#libraryproperties-file-format
         (Rule LP038)                                                                                          

Linter results for project: 0 ERRORS, 1 WARNINGS

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Linting sketch in millis64/examples/micros64_demo

Linter results for project: no errors or warnings

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Linting sketch in millis64/examples/millis64_demo

Linter results for project: no errors or warnings

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Linting sketch in millis64/examples/millis64_performance

Linter results for project: no errors or warnings

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Linter results for projects: 0 ERRORS, 1 WARNINGS

[github-actions]

Your submission has now been accepted! Thanks for your contribution to the Arduino Library Manager index.

The library(s) will be available for installation via Library Manager within a day's time.

You can check the logs from the Library Manager indexer for your library(s) here:
http://downloads.arduino.cc/libraries/logs/github.com/RobTillaart/millis64/