[Superposition] Change Bloch sphere link

Superposition/Workbook_Superposition_Part2.ipynb

@@ -789,7 +789,7 @@
     "Looking at the other two gates, $R_x$ gate rotates the qubit state around the X axis and thus takes the qubit state out of the X-Z plane, introducing complex coefficients. $R_z$ gate rotates the qubit state around the Z axis, so it doesn't modify the $|0\\rangle$ state which lies on the Z axis.\n",
     "<br/><br/> \n",
     "In case you are surprised to see the $|0\\rangle$ and $|1\\rangle$ vectors 180 degrees apart from each other, the Bloch sphere represents orthogonal vectors as opposite points on the sphere.  \n",
-    "You can learn more about the Bloch sphere from <a href=\"http://www.vcpc.univie.ac.at/~ian/hotlist/qc/talks/bloch-sphere.pdf\">this presentation</a>.</td>\n",
+    "You can learn more about the Bloch sphere in <a href=\"https://en.wikipedia.org/wiki/Bloch_sphere\">this article</a>.</td>\n",
     "        <td style=\"text-align:center; font-size: 14px; background-color:white; border:0\"><img src=\"./img/blochsphere.png\"/></td> \n",
     "      </tr>    \n",
     "</table>"