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

@@ -12,11 +12,11 @@ DHT	KEYWORD1
 # Methods and Functions (KEYWORD2)
 ###########################################
 
-begin KEYWORD2
-readTemperature KEYWORD2
-convertCtoF KEYWORD2
-convertFtoC KEYWORD2
-computeHeatIndex KEYWORD2
-readHumidity KEYWORD2
-read KEYWORD2
+begin	KEYWORD2
+readTemperature	KEYWORD2
+convertCtoF	KEYWORD2
+convertFtoC	KEYWORD2
+computeHeatIndex	KEYWORD2
+readHumidity	KEYWORD2
+read	KEYWORD2