doc/go1.9: fix typo in crypto/x509 of "Minor changes to the library".

Change-Id: I79fee40370dc6758ca1869227f076c61d07f21a5
Reviewed-on: https://go-review.googlesource.com/57390Reviewed-by: Avelino <t@avelino.xxx>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.9.html

@@ -490,7 +490,7 @@ version of gccgo.
 
     <p><!-- CL 36696 -->
 
-    If any SAN extension, including with no DSN names, is present
+    If any SAN extension, including with no DNS names, is present
     in the certificate, then the Common Name from
     <a href="/pkg/crypto/x509/#Certificate.Subject"><code>Subject</code></a> is ignored.
     In previous releases, the code tested only whether DNS-name SANs were