Fix link to Erbium's CoAP engine website

The original site is no longer available, therefore linking to
latest version mirrored on archive.org.

Signed-off-by: Reto Schneider <code@reto-schneider.ch>

README.md

@@ -8,7 +8,7 @@ Developers mailing list: https://dev.eclipse.org/mailman/listinfo/wakaama-dev
     -+- core                   (the LWM2M engine)
      |    |
      |    +- er-coap-13        (Slightly modified Erbium's CoAP engine from
-     |                          http://people.inf.ethz.ch/mkovatsc/erbium.php
+     |                          https://web.archive.org/web/20180316172739/http://people.inf.ethz.ch/mkovatsc/erbium.php)
      |
      +- tests                  (test cases)
      |