doc/go1.3.html: fix typo

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/92440043

doc/go1.3.html

@@ -77,7 +77,7 @@ As of Go 1.3, support for Go on NetBSD requires NetBSD 6.0 or above.
 <h3 id="openbsd">Support for OpenBSD</h3>
 
 <p>
-As of Go 1.3, support for Go on NetBSD requires OpenBSD 5.5 or above.
+As of Go 1.3, support for Go on OpenBSD requires OpenBSD 5.5 or above.
 </p>
 
 <h3 id="plan9">Support for Plan 9</h3>