TL: another mathjax fix

Author: tlunet

docs/notebooks/22_nodes.ipynb

@@ -176,11 +176,11 @@
     "The node computation process relies on the **three term recurrence coefficients** associated to each polynomial basis :\n",
     "\n",
     "$$\n",
-    "\\begin{gather}\n",
+    "\\begin{aligned}\n",
     "\\forall j \\in \\mathbb{N}, \\quad\n",
     "\\pi_{j+1}(t) = (t-\\alpha_j)\\pi_{j}(t) - \\beta_j \\pi_{j-1}(t), \\\\\n",
     "\\pi_{-1}(t) = 0, \\quad \\pi_0(t) = 1.\n",
-    "\\end{gather}\n",
+    "\\end{aligned}\n",
     "$$\n",
     "\n",
     "Those coefficients are know analytically for each polynomial basis (_i.e_ node type), \n",