Use correct field separator in keywords.txt

[per1234]

The Arduino IDE requires the use of a single true tab separator between the keyword name and identifier. When spaces are used rather than a true tab the keyword is not highlighted.

Reference:
https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5:-Library-specification#keywords

[ericbarrionuevo]

This is the specification for the Arduino library format, to be used with Arduino IDE 1.5.x onwards.

rev.1 has been implemented starting with Arduino IDE version 1.5.3 (now superseded by rev.2)
rev.2 will be implemented starting from Arduino IDE version 1.5.6
rev.2.1 will be implemented starting from Arduino IDE version 1.6.10
rev.2.2 will be implemented starting from Arduino IDE version 1.8.10

This new library format is intended to be used in tandem with Library Manager, available since Arduino IDE 1.6.2. The Library Manager allows users to automatically download and install libraries needed in their projects, with an easy to use graphical interface in the Arduino IDE/Arduino IDE 2.0 and Arduino Web Editor as well as arduino-cli lib.