Use correct field separator in keywords.txt (#31)

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

@@ -39,7 +39,7 @@ FC_WRITE_MULTIPLE_REGISTERS	LITERAL1
 CB_READ_COILS	LITERAL1
 CB_READ_DISCRETE_INPUTS LITERAL1
 CB_READ_HOLDING_REGISTERS	LITERAL1
-CB_READ_INPUT_REGISTERS LITERAL1
+CB_READ_INPUT_REGISTERS	LITERAL1
 CB_WRITE_COILS	LITERAL1
 CB_WRITE_HOLDING_REGISTERS	LITERAL1
 COIL_OFF	LITERAL1