Fix background color for terminal256

At least on my OS X 10.8 terminal, the background color of `Text::Whitespace` appears as violet instead of dark:

![screenshot_01](https://f.cloud.github.com/assets/1037292/1558255/f8aa03b2-4f77-11e3-9e25-8c4326e56e44.png)
(Second line is after the patch.)

Looking at the parameters table on [Wikipedia](https://en.wikipedia.org/wiki/ANSI_escape_code#CSI_codes), it seems that code 48 is the correct one, not 45.