doc/effective_go.html: replace tab with spaces.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/4609042

doc/effective_go.html

@@ -350,7 +350,7 @@ Both names read well in practice:
 <pre>
 owner := obj.Owner()
 if owner != user {
-	obj.SetOwner(user)
+    obj.SetOwner(user)
 }
 </pre>