Fix grammatical error in tutorial description (#708)

Author: ndem0

tutorials/tutorial9/tutorial.ipynb

@@ -8,7 +8,7 @@
     "\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)\n",
     "\n",
-    "This tutorial demonstrates how to solve a one-dimensional Helmholtz equation with periodic boundary conditions (PBC) using Physics-Informed Neural Networks (PINNs).  \n",
+    "This tutorial demonstrate how to solve a one-dimensional Helmholtz equation with periodic boundary conditions (PBC) using Physics-Informed Neural Networks (PINNs).  \n",
     "We will use standard PINN training, augmented with a periodic input expansion as introduced in [*An Expert’s Guide to Training Physics-Informed Neural Networks*](https://arxiv.org/abs/2308.08468).\n",
     "\n",
     "Let's start with some useful imports:\n"