install.html: minor sentence fix

R=r, r2
CC=golang-dev
https://golang.org/cl/2312041

doc/install.html

@@ -93,7 +93,7 @@ installed as part of
 <h2 id="mercurial">Install Mercurial, if needed</h2>
 
 <p>
-To perform the next step you must have Mercurial installed. (Check that you  have an <code>hg</code> command.) This suffices to install Mercurial most systems:
+To perform the next step you must have Mercurial installed. (Check that you  have an <code>hg</code> command.) This suffices to install Mercurial on most systems:
 </p>
 <pre>
 sudo easy_install mercurial