Mongo Express: Update docs of mongo-express

[berviantoleo]

Update the mongo-express docs.

Reference:

https://github.com/mongo-express/mongo-express-docker/blob/master/README.md

Related issue:

mongo-express/mongo-express-docker#106

[tianon]

In CI, our markdownfmt I think found a case of tabs vs spaces, and I've left a couple suggestions for using the templating to inject the image name (so it works properly on the architecture-specific repos too)

[tianon]

You might also appreciate ./markdownfmt.sh -w mongo-express/content.md (will run markdownfmt in a container and just write the result directly to your local file which you can then commit).