doc: fix typo in Go 1.9 release notes

Change-Id: I78443d5874a7667fbac78eac188d402227373da3
Reviewed-on: https://go-review.googlesource.com/45813Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.9.html

@@ -797,7 +797,7 @@ CL 40331: https://golang.org/cl/40331: cmd/link,runtime/cgo: enable PT_TLS gener
     </p>
 
     <p><!-- CL 44832 -->
-      The included <code>tzdata</code> timezone database has bene
+      The included <code>tzdata</code> timezone database has been
       updated to version 2017b. As always, it is only used if the
       system does not already have the database available.
     </p>