doc: make chart.apis.google.com link not clickable
The example in the 'A web server' section of the effective Go document
uses Google's image charts API (at chart.apis.google.com).
The service is now deprecated (see developers.google.com/chart/image),
and visiting http://chart.apis.google.com gives a 404. The endpoint is
still active, so the Go code in the example still works, but there's
no point in making the link clickable by the user if the page returns
a 404.