doc: fix typo in go1.9 release notes

Change-Id: I64d32fd308f2b7968dfaf11cdc85ab1952170868
Reviewed-on: https://go-review.googlesource.com/45891Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.9.html

@@ -610,7 +610,7 @@ CL 40331: https://golang.org/cl/40331: cmd/link,runtime/cgo: enable PT_TLS gener
 <dl id="os"><dt><a href="/pkg/os/">os</a></dt>
   <dd>
     <p><!-- CL 36800 -->
-      The <code>os</code>package now uses the internal runtime poller
+      The <code>os</code> package now uses the internal runtime poller
       for file I/O.
       This reduces the number of threads required for read/write
       operations on pipes, and eliminates races when one goroutine