doc/effective_go: another doubled word

It's clear that that should never happen.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/8798045

doc/effective_go.html

@@ -1570,7 +1570,7 @@ _, present := timeZone[tz]
 <p>
 To delete a map entry, use the <code>delete</code>
 built-in function, whose arguments are the map and the key to be deleted.
-It's safe to do this this even if the key is already absent
+It's safe to do this even if the key is already absent
 from the map.
 </p>
 <pre>