Examples: fix invalid html

The W3C Validator service reveals that all examples of the gobyexample site contain invalid html:

See e.g. the go routines example

This PR fixes this. It also adds a lang attribute to the html tag, as advised by the validator.

The same service gives me >200 errors on HN and same for reddit.com (just two sites I randomly checked) :-)

Have to think about it a bit, as I'm not sure I see the point in this.