Render docs.

templates/docs/signalling_failures.html

@@ -4,7 +4,7 @@ Requesting the <code>/fail</code> URL will immediately mark the check as "down".
 You can use this feature to minimize the delay from your monitored service failing
 to you getting a notification.</p>
 <h2>Shell Scripts</h2>
-<p>The below shell script sends sends either a "success" or "failure" ping depending on
+<p>The below shell script sends either a "success" or "failure" ping depending on
 command's (certbot in this example) exit code:</p>
 <div class="highlight"><pre><span></span><code><span class="ch">#!/bin/sh</span>