[Superposition] Fix filename capitalization in workbook (#219)

Binder is sensitive to capitalization of file names in links; this change converts image files' extensions to lowercase and fixes the links to match

Author: jimcristofono

Superposition/Workbook_Superposition.ipynb

@@ -493,8 +493,8 @@
     "        <td style=\"text-align:center; border:1px solid\">$\\text{CNOT} = \\begin{bmatrix} 1 & 0 & 0 & 0 \\\\ 0 & 1 & 0 & 0 \\\\ 0 & 0 & 0 & 1 \\\\ 0 & 0 & 1 & 0 \\end{bmatrix}$</td>\n",
     "        <td style=\"text-align:center; border:1px solid\"><img src=\"./img/CNOTGateCircuit.png\"/></td>    \n",
     "      </tr>      \n",
-    "</table>\n",
-    "<center>The matrix and circuit representation of CNOT</center>\n",
+    "</table> <br>\n",
+    "<center>The matrix and circuit representation of CNOT</center><br>\n",
     "\n",
     "If we apply the CNOT gate to the state $\\frac{1}{\\sqrt2} (|00\\rangle + |10\\rangle)$, taking the first qubit as the control and the second one as target, we'll get exactly the desired goal state. \n",
     "<img src=\"./img/Task6OutputHadamardasControl.png\" width=\"200\"/>\n",
@@ -511,7 +511,7 @@
     "    <tr>\n",
     "        <td style=\"text-align:left; border:1px solid\">1. Apply a Hadamard gate to the first qubit. <br/> 2. Applying a CNOT with first qubit as control and second qubit as target.</td>\n",
     "        <td style=\"text-align:center; border:1px solid; font-bold; font-size: 16px; \">$\\frac{1}{\\sqrt2} (|00\\rangle + |11\\rangle)$</td>\n",
-    "        <td style=\"text-align:center; border:1px solid\"><img src=\"./img/Task6HadamardCNOTCircuit.PNG\"/></td>\n",
+    "        <td style=\"text-align:center; border:1px solid\"><img src=\"./img/Task6HadamardCNOTCircuit.png\"/></td>\n",
     "      </tr>      \n",
     "</table>\n",
     "\n",
@@ -617,7 +617,7 @@
     "    <col width=50>\n",
     "    <col width=300>\n",
     "    <tr bgcolor=\"white\">\n",
-    "        <td bgcolor=\"white\" style=\"text-align:center\"><img src=\"./img/Task6HadamardCNOTCircuit.PNG\"/></td>\n",
+    "        <td bgcolor=\"white\" style=\"text-align:center\"><img src=\"./img/Task6HadamardCNOTCircuit.png\"/></td>\n",
     "        <td bgcolor=\"white\" style=\"text-align:center;font-size: 30px\">$\\Longrightarrow$</td>\n",
     "        <td bgcolor=\"white\" style=\"text-align:center;font-size: 20px\">$\\frac{1}{\\sqrt2} \\big(|00\\rangle + |11\\rangle\\big)$</td>\n",
     "      </tr>      \n",