Docs: fix second order derivative formula

docs/source/3_iraman.ipynb

@@ -39,11 +39,11 @@
     "The same theory we saw in previous tutorials applies here. Note that for computing _second order_ derivatives of forces we need a slightly different formula:\n",
     "\n",
     "\\begin{equation}\n",
-    "    \\left . \\frac{\\partial f(x)}{\\partial x} \\right|_{x=0}\n",
+    "    \\left . \\frac{\\partial^2 f(x)}{\\partial x^2} \\right|_{x=0}\n",
     "    =\n",
     "    \\frac{1}{h^2}\n",
     "    \\left [\n",
-    "        f(h) -2f(0) +f(-2h)\n",
+    "        f(h) -2f(0) +f(-h)\n",
     "    \\right ]\n",
     "    + \\mathcal{O}(h^2)\n",
     "\\end{equation}\n",