Fix link on jupyter notebook (#4409)

Fix link on jupyter notebook.

Author: seunomonije

cirq-web/circuit_example.ipynb

@@ -44,7 +44,7 @@
     "\n",
     "<table class=\"tfo-notebook-buttons\" align=\"left\">\n",
     "  <td>\n",
-    "    <a target=\"_blank\" href=\"https://colab.research.google.com/github/quantumlib/Cirq/blob/master/master/cirq-web/circuit-example.ipynb\"><img src=\"https://quantumai.google/site-assets/images/buttons/colab_logo_1x.png\" />Run in Google Colab</a>\n",
+    "    <a target=\"_blank\" href=\"https://colab.research.google.com/github/quantumlib/Cirq/blob/master/cirq-web/circuit_example.ipynb\"><img src=\"https://quantumai.google/site-assets/images/buttons/colab_logo_1x.png\" />Run in Google Colab</a>\n",
     "  </td>\n",
     "  <td>\n",
     "    <a target=\"_blank\" href=\"https://github.com/quantumlib/Cirq/blob/master/cirq-web/circuit-example.ipynb\"><img src=\"https://quantumai.google/site-assets/images/buttons/github_logo_1x.png\" />View source on GitHub</a>\n",