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.

Change the element to `<code>`.

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

doc/effective_go.html

@@ -3588,8 +3588,7 @@ That's left as an exercise for the reader.
 <p>
 Let's finish with a complete Go program, a web server.
 This one is actually a kind of web re-server.
-Google provides a service at
-<a href="http://chart.apis.google.com">http://chart.apis.google.com</a>
+Google provides a service at <code>chart.apis.google.com</code>
 that does automatic formatting of data into charts and graphs.
 It's hard to use interactively, though,
 because you need to put the data into the URL as a query.