doc/go1.7.html: typo fix; replace "," at end of sentence with "."

Signed-off-by: Steven Phillips <steve@tryingtobeawesome.com>

Change-Id: Ie7c3253a5e1cd43be8fa12bad340204cc6c5ca76
Reviewed-on: https://go-review.googlesource.com/23677Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.7.html

@@ -973,7 +973,7 @@ For compatibility with older mail parsers,
 the address encoder, namely
 <a href="/pkg/net/mail/#Address"><code>Address</code></a>'s
 <a href="/pkg/net/mail/#Address.String"><code>String</code></a> method,
-continues to escape all UTF-8 text following <a href="https://tools.ietf.org/html/rfc5322">RFC 5322</a>,
+continues to escape all UTF-8 text following <a href="https://tools.ietf.org/html/rfc5322">RFC 5322</a>.
 </p>
 </dd>