doc: fix typo in install-source.html

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/13892043

doc/install-source.html

@@ -223,7 +223,7 @@ set up a work environment in which to build and test Go code.
 <h2 id="tools">Install additional tools</h2>
 
 <p>
-The source code for seeral Go tools (including <a href="/cmd/godoc/">godoc</a>)
+The source code for several Go tools (including <a href="/cmd/godoc/">godoc</a>)
 is kept in <a href="https://code.google.com/p/go.tools">the go.tools repository</a>.
 To install all of them, run the <code>go</code> <code>get</code> command:
 </p>