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

@@ -6,7 +6,7 @@
 # Datatypes (KEYWORD1)
 #######################################
 
-LiquidCrystal_PCF8574 KEYWORD1
+LiquidCrystal_PCF8574	KEYWORD1
 
 #######################################
 # Methods and Functions (KEYWORD2)