Fix broken link in external-dns.html (#2045)

management/templates/external-dns.html

@@ -38,7 +38,7 @@ <h2>External DNS</h2>
 <p class="alert" role="alert">
  <span class="glyphicon glyphicon-info-sign"></span>
  You may encounter zone file errors when attempting to create a TXT record with a long string.
- <a href="http://tools.ietf.org/html/rfc4408#section-3.1.3">RFC 4408</a> states a TXT record is allowed to contain multiple strings, and this technique can be used to construct records that would exceed the 255-byte maximum length.
+ <a href="https://tools.ietf.org/html/rfc4408#section-3.1.3">RFC 4408</a> states a TXT record is allowed to contain multiple strings, and this technique can be used to construct records that would exceed the 255-byte maximum length.
  You may need to adopt this technique when adding DomainKeys. Use a tool like <code>named-checkzone</code> to validate your zone file.
 </p>