documentation: Mention Google Code Prettify in licenses

Although we have been embedding this JavaScript library since version
2.0.17 we completely failed to update the notice documentation
listing its use.  Correct that oversight.

Change-Id: I3bcbaa3c1a442fd45f8a94b2d2c12b678080c72c
Signed-off-by: Shawn O. Pearce <sop@google.com>
1 file changed