Notebook typos and bad expression (#450)

* Correcting typos

* Correct typo and inhomogeneous expression

* Correct value from _mock_device.py

* Changing C3 units

* Deleting space after $

tutorials/quantum_simulation/Spin chain of 3 atoms in XY mode.ipynb

@@ -17,7 +17,7 @@
     "H_{XY} = \\frac{1}{2}\\sum_{i<j} \\frac{C_3}{R_{ij}^3}(\\sigma^+_i \\sigma^-_j + \\sigma^-_i \\sigma^+_j) \n",
     "$$\n",
     "\n",
-    "where $\\sigma^{±} = (\\sigma^x ± i\\sigma^y)/2$, $R_{ij}$ is the distance between atom $i$ and atom $j$, and $C_3$ is a coefficient dependent on the device and taken to 3070 MHz $\\cdot \\mu m^{-3}$ for the `MockDevice`.\n",
+    "where $\\sigma^{±} = (\\sigma^x ± i\\sigma^y)/2$, $R_{ij}$ is the distance between atom $i$ and atom $j$, and $C_3$ is a coefficient dependent on the device and taken as  $\\frac{C_3}{\\hbar} = 3700 \\frac{\\mu m^3}{\\mu s}$ for the `MockDevice`.\n",
     "\n",
     "The final hamiltonian of the system is \n",
     "\n",