Update install-mongodb-on-ubuntu.txt

Use URI prefix and port 80 for keyserver.  Should be generically good when called from inside more restrictive network environments and harmless elsewhere.

See:  http://stackoverflow.com/questions/13112400/gpgkeys-key-7f0ceb10-not-found-on-keyserver-while-installing-mongodb-on-ubunt

Signed-off-by: Sam Kleinman <samk@10gen.com>