doc: replace "private" with "unexported" in Effective Go.

R=r
CC=golang-codereviews
https://golang.org/cl/53720043

doc/effective_go.html

@@ -288,7 +288,7 @@ var (
 </pre>
 
 <p>
-Even for private names, grouping can also indicate relationships between items,
+Grouping can also indicate relationships between items,
 such as the fact that a set of variables is protected by a mutex.
 </p>