[release-branch.go1.9] doc/go1.9: fix typo in crypto/x509 of "Minor changes to the library".

Backport of https://golang.org/cl/57390 to 1.9 release branch.

Change-Id: Ieea5a048732db7ee5dc5cf13f06e11ca4f5313cc
Reviewed-on: https://go-review.googlesource.com/57450Reviewed-by: Keith Randall <khr@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