add I2C bus to README example

Author: dhalbert

README.rst

@@ -20,6 +20,8 @@ Currently there is only one such class: ``I2CPCF8574Interface``.
 
 .. code-block:: python
 
+    import board
+    
     from lcd.lcd import LCD
     from lcd.i2c_pcf8574_interface import I2CPCF8574Interface
 
@@ -28,7 +30,7 @@ Currently there is only one such class: ``I2CPCF8574Interface``.
     # Talk to the LCD at I2C address 0x27.
     # The number of rows and columns defaults to 4x20, so those
     # arguments could be omitted in this case.
-    lcd = LCD(I2CPCF8574Interface(0x27), num_rows=4, num_cols=20)
+    lcd = LCD(I2CPCF8574Interface(board.I2C(), 0x27), num_rows=4, num_cols=20)
 
     lcd.print("abc ")
     lcd.print("This is quite long and will wrap onto the next line automatically.")