docs/manual/getting: fix dead link

When searching for the official git repository in the documentation,
readers are forwarded to the buildroot website Download page with a link
pointing to buildroot.org/download, which returns a 404.

Fix the dead link by adding the missing .html suffix.

Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

docs/manual/getting.adoc

@@ -32,5 +32,5 @@ vagrant up
 
 If you want to follow development, you can use the daily snapshots or
 make a clone of the Git repository. Refer to the
-http://buildroot.org/download[Download page] of the Buildroot website
+http://buildroot.org/download.html[Download page] of the Buildroot website
 for more details.