fix link tutorials

Author: dario-coscia

tutorials/tutorial21/tutorial.ipynb

@@ -8,7 +8,7 @@
    "source": [
     "# Tutorial: Introductory Tutorial: Supervised Learning with PINA\n",
     "\n",
-    "[![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/mathLab/PINA/blob/master/tutorials/tutorial20/tutorial.ipynb)\n",
+    "[![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/mathLab/PINA/blob/master/tutorials/tutorial21/tutorial.ipynb)\n",
     "\n",
     "\n",
     "> ##### ⚠️ ***Before starting:***\n",

tutorials/tutorial8/tutorial.ipynb

@@ -7,7 +7,7 @@
    "source": [
     "# Tutorial: Reduced Order Modeling with POD-RBF and POD-NN Approaches for Fluid Dynamics\n",
     "\n",
-    "[![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/mathLab/PINA/blob/master/tutorials/tutorial9/tutorial.ipynb)"
+    "[![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/mathLab/PINA/blob/master/tutorials/tutorial8/tutorial.ipynb)"
    ]
   },
   {