Improve the example in the python tutorial. (#1019)

The python code-generation example previously failed to handle
wrapped expressions like: "(a * (b + c))".

Fix typo in nmodl python tutorial.

docs/notebooks/nmodl-python-tutorial.ipynb

@@ -682,6 +682,11 @@
     "        self.pycode += \" \" * 4 * self.indent + \"else:\\n\"\n",
     "        node.get_statement_block().accept(self)\n",
     "\n",
+    "    def visit_wrapped_expression(self, node):\n",
+    "        self.pycode += \"(\"\n",
+    "        node.visit_children(self)\n",
+    "        self.pycode += \")\"\n",
+    "\n",
     "    def visit_binary_expression(self, node):\n",
     "        lhs = node.lhs\n",
     "        rhs = node.rhs\n",
@@ -701,7 +706,7 @@
     "        self.pycode += node.name.get_node_name()\n",
     "\n",
     "    def visit_integer(self, node):\n",
-    "        self.pycode += nmod.to_nmodl(node)\n",
+    "        self.pycode += nmodl.to_nmodl(node)\n",
     "\n",
     "    def visit_double(self, node):\n",
     "        self.pycode += nmodl.to_nmodl(node)"