doc: mention go/printer instead of container/vector in effective go

Fixes #1876.

R=golang-dev, bradfitz, gri, r
CC=golang-dev
https://golang.org/cl/4538093

doc/effective_go.html

@@ -297,9 +297,9 @@ determines just which package is being used.
 <p>
 Another convention is that the package name is the base name of
 its source directory;
-the package in <code>src/pkg/container/vector</code>
-is imported as <code>"container/vector"</code> but has name <code>vector</code>,
-not <code>container_vector</code> and not <code>containerVector</code>.
+the package in <code>src/pkg/encoding/base64</code>
+is imported as <code>"encoding/base64"</code> but has name <code>base64</code>,
+not <code>encoding_base64</code> and not <code>encodingBase64</code>.
 </p>
 
 <p>