Fixed '\mathcal{I}' latex error when it should have just been 'I'

fdm-jupyter-book/notebooks/01_vib/vib_undamped.ipynb

@@ -828,8 +828,7 @@
    "metadata": {},
    "source": [
     "The error analysis in the section [Analysis of the numerical scheme](#vib:ode1:analysis) is quite\n",
-    "detailed and suggests that $r=2$.\n",
-    "mathcal{I}_t is also a intuitively reasonable result, since we used a\n",
+    "detailed and suggests that $r=2$. It is also a intuitively reasonable result, since we used a\n",
     "second-order accurate finite difference approximation $[D_tD_tu]^n$ to\n",
     "the ODE and a second-order accurate finite difference formula for the\n",
     "initial condition for $u^{\\prime}$.\n",