fix a typo in NEP tutorial

examples/nep_potentials/PbTe/train/nep_tutorial.ipynb

@@ -36,7 +36,7 @@
     "- This is the training data set for PbTe as used in the NEP1 paper: https://doi.org/10.1103/PhysRevB.104.104309\n",
     "- It is also the training data set for PbTe as used in the NEP2 paper: https://arxiv.org/abs/2109.10643\n",
     "- In this example, there are only energy and force training data. In general, one can also have virial in `train.in`.\n",
-    "- We used the VAPS code to calculate the training data, but `nep` does not care about this. You can generate the data in `train.in` using any method that works for you. For example, before doing new DFT calculations, you can first check if there are training data publicly available and then try to convert them into the format as required by `train.in`. \n",
+    "- We used the VASP code to calculate the training data, but `nep` does not care about this. You can generate the data in `train.in` using any method that works for you. For example, before doing new DFT calculations, you can first check if there are training data publicly available and then try to convert them into the format as required by `train.in`. \n",
     "- For simplicity, the `test.in` file has the same data as in the `train.in` file."
    ]
   },