Update doc (fixes #301)

doc/python-usage.rst

@@ -12,13 +12,14 @@ In this example, we are using an I2C interface with a ssd1306 display.
 
 .. code:: python
 
-  from luma.core.interface.serial import i2c, spi, parallel, pcf8574
+  from luma.core.interface.serial import i2c, spi, pcf8574
+  from luma.core.interface.parallel import bitbang_6800
   from luma.core.render import canvas
   from luma.oled.device import ssd1306, ssd1309, ssd1325, ssd1331, sh1106, ws0010
 
   # rev.1 users set port=0
   # substitute spi(device=0, port=0) below if using that interface
-  # substitute parallel(RS=7, E=8, PINS=[25,24,23,27]) below if using that interface
+  # substitute bitbang_6800(RS=7, E=8, PINS=[25,24,23,27]) below if using that interface
   serial = i2c(port=1, address=0x3C)
 
   # substitute ssd1331(...) or sh1106(...) below if using that device