doc: fix link to Mercurial download

Oddly, 404s with the trailing slash.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/90790045

doc/install-source.html

@@ -95,7 +95,7 @@ have an <code>hg</code> command.)
 <p>
 If you do not have a working Mercurial installation,
 follow the instructions on the
-<a href="http://mercurial.selenic.com/downloads/">Mercurial downloads</a> page.
+<a href="http://mercurial.selenic.com/downloads">Mercurial downloads</a> page.
 </p>
 
 <p>