Fix display consistency of JavaScript code

Signed-off-by: Sam Kleinman <samk@10gen.com>
Update sharding-introduction.txt

/images/sharded-cluster-production-architecture.rst needs to be edited to reflect "Query Router" instead of "Router". It also has a caption that should be updated along the same lines.

Signed-off-by: Sam Kleinman <samk@10gen.com>
minor: tweaking images for router -> query router