docs (challenge): add link to reference for resilience level

docs/challenges/dynamical_decoupling.ipynb

@@ -577,7 +577,7 @@
    "id": "1bd78683-4503-49c5-ba5e-0d2bf5002ca8",
    "metadata": {},
    "source": [
-    "Vary the number of qubits and the density of the circuit. For each case, apply DD pulse and use measurement error mitigation by setting `resilience_level` to 1 (measurement error mitigation) and 2 (zero noise extrapolation). For each instance \n",
+    "Vary the number of qubits and the density of the circuit. For each case, apply DD pulse and use measurement error mitigation by setting `resilience_level` to 1 and 2 <a href='https://docs.quantum.ibm.com/run/configure-error-mitigation'>[1]</a>. For each instance \n",
     "\n",
     "<ol>\n",
     "    <li>determine whether the best DD pulse changes from the previous scenario with no mitigation</li>\n",