remove align to so $$ do not show on the rendered docs

Author: mberz

chapter2/helmholtz_code.ipynb

@@ -213,9 +213,7 @@
     "\n",
     "With these, we can compute the surface impedance, that in the case of a rigid passive absorber placed on a rigid wall is given by the formula\n",
     "$$\n",
-    "\\begin{align}\n",
     "Z_s = -j Z_c cot(k_c d).\n",
-    "\\end{align}\n",
     "$$\n",
     "\n",
     "Let's create a function to compute it.\n",