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

Author: per1234

keywords.txt

@@ -15,9 +15,9 @@ THERM	KEYWORD1
 begin	KEYWORD2
 end	KEYWORD2
 
-readTemperature KEYWORD2
+readTemperature	KEYWORD2
 readReferenceTemperature	KEYWORD2
 
 #######################################
 # Constants
-#######################################
+#######################################