doc: fix typo in install-source.html.

Change-Id: I6ea7339ff9412111319e46f2962c6b3880987a1e
Reviewed-on: https://go-review.googlesource.com/18174Reviewed-by: Minux Ma <minux@golang.org>

doc/install-source.html

@@ -139,7 +139,7 @@ and used as <code>GOROOT_BOOTSTRAP</code> to bootstrap a local build.
 </p>
 
 <p>
-To use gccgo, you need to arrange for <code>$GOROOT_BOOSTRAP/bin/go</code> to be
+To use gccgo, you need to arrange for <code>$GOROOT_BOOTSTRAP/bin/go</code> to be
 the go tool that comes as part of gccgo 5. For example on Ubuntu Vivid:
 </p>