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.

Change the element to `<code>`.

Change-Id: Ie67f4723cfa636e3dc1460507055b6bbb2b0970c
Reviewed-on: https://go-review.googlesource.com/109576Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>