Use correct field separator in keywords.txt

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

keywords.txt

@@ -14,7 +14,7 @@ avdweb_VirtualDelay	KEYWORD1
 
 start	KEYWORD2
 elapsed	KEYWORD2
-DO_ONCE KEYWORD2
+DO_ONCE	KEYWORD2
 
 #######################################
 # Instances (KEYWORD2)