example/font: use an "example" build tag instead of "ignore".

The golang-dev discussion is at
https://groups.google.com/d/topic/golang-dev/qKpxZiNABcY/discussion

Change-Id: I55d626fd9b4c5585d11c585a8553fb5184d0ace7
Reviewed-on: https://go-review.googlesource.com/21788
Reviewed-by: Andrew Gerrand <adg@golang.org>

example/font/main.go

@@ -2,10 +2,11 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build ignore
+// +build example
 //
 // This build tag means that "go install golang.org/x/image/..." doesn't
-// install this example program. Use "go run main.go" to run it.
+// install this example program. Use "go run main.go" to run it or "go install
+// -tags=example" to install it.
 
 // Font is a basic example of using fonts.
 package main