claim.html: only show the main key fingerprint

Without this additional command, then the subkey fingerprints will be
listed as well, making the user not sure about which fingerprint to use.

Closes #47.

dm/templates/dm/claim.html

@@ -16,7 +16,7 @@ fix this situation.</p>
 and I would like to find out using your GPG key. Please enter the fingerprint
 in this form (just the hexadecimal digits, as in:</p>
 
-<pre>LANG=en gpg --fingerprint $YOUR_KEYID | sed -nre 's/Key fingerprint =//p'</pre>
+<pre>LANG=en gpg --fingerprint $YOUR_KEYID | sed -nre 's/Key fingerprint =//p' | head -n1</pre>
 
 <form action="" method="POST">{% csrf_token %}
 {{form.as_p}}