Fix an equation which does not display correctly in nbsphinx. (#3008)

docs/tutorials/variational_algorithm.ipynb

@@ -260,7 +260,7 @@
       "source": [
         "Another important concept here is that the rotation gate is specified in *half turns* ($ht$). For a rotation about `X`, the gate is:\n",
         "\n",
-        "$ \\cos(ht * \\pi) I + i \\sin(ht * \\pi) X$\n",
+        "$\\cos(ht * \\pi) I + i \\sin(ht * \\pi) X$\n",
         "\n",
         "There is a lot of freedom defining a variational ansatz. Here we will do a variation on a [QAOA strategy](https://arxiv.org/abs/1411.4028) and define an ansatz related to the problem we are trying to solve.\n",
         "\n",