doc/install.html: FreeBSD 8 and higher only are supported

Fixes #7188

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://golang.org/cl/88280044

doc/install.html

@@ -14,7 +14,7 @@
 
 <p>
 <a href="http://code.google.com/p/go/downloads" target="_blank">Official binary
-distributions</a> are available for the FreeBSD, Linux, Mac OS X (Snow Leopard
+distributions</a> are available for the FreeBSD (release 8 and above), Linux, Mac OS X (Snow Leopard
 and above), and Windows operating systems and the 32-bit (<code>386</code>) and
 64-bit (<code>amd64</code>) x86 processor architectures.
 </p>