doc: fix typo in effective_go.html

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7548045

doc/effective_go.html

@@ -2466,7 +2466,7 @@ only to have them be needed again later.
 The blank identifier provides a workaround.
 </p>
 <p>
-This half-written program is has two unused imports
+This half-written program has two unused imports
 (<code>fmt</code> and <code>io</code>)
 and an unused variable (<code>fd</code>),
 so it will not compile, but it would be nice to see if the