Fix image for eBook

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

Author: alexellis

_layouts/page.html

@@ -31,7 +31,7 @@ <h2 class="subtitle is-size-4">
 	<section class="gumroad-cta">
 		<h3 class="gumroad-cta-title">Checkout the official eBook and video workshop for OpenFaaS</h3>
 			<a href="http://store.openfaas.com/l/serverless-for-everyone-else">
-			<img src="https://www.alexellis.io/serverless.png" alt="Serverless eBook cover">
+			<img src="/images/serverless-for-everyone-else.png" alt="Serverless eBook cover">
 			</a>
 			<p>Learn use-cases for open-source serverless functions and how to integrate them into your existing workflows.</p>
 			<p>Through the hands-on exercises you will build your own JavaScript functions using Node.js. You'll add custom npm modules, integrate with other HTTP APIs, enable authentication and monitor the endpoints with Grafana.</p>