docs/website: fix favicon

When the favicon image was added in f26e61319f3f (docs/website: add
favicon.png), it was added to a different directory then where the header's
icon link points. This causes the favicon to fail to load with 404.

While we are here, remove the "shortcut" rel attribute as it is non-standard
and it's recommended not to use it[1].

[1] https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/rel#sect4

Signed-off-by: Brandon Maier <brandon.maier@collins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 8ad1a2eaa5fa6c5eaa6614b007b968223e49448e)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/website/header.html

@@ -6,7 +6,7 @@
 	<meta name="viewport" content="width=device-width, initial-scale=1.0">
 	<meta name="Buildroot" content="">
 	<meta name="angelo.compagnucci@gmail.com" content="">
-	<link rel="shortcut icon" href="images/favicon.png">
+	<link rel="icon" href="favicon.png">
 
 	<title>Buildroot - Making Embedded Linux Easy</title>