fix a link

examples/11_NEP_potential_PbTe/tutorial.ipynb

@@ -31,7 +31,7 @@
     "- The ``train.xyz``/``test.xyz`` file should be prepared according to the [documentation](https://gpumd.org/nep/input_files/).\n",
     "- For the example in this tutorial, we have already prepared the ``train.xyz`` and ``test.xyz`` files in the current folder, which contain 25 and 300 structures, respectively.\n",
     "- In this example, there are only energy and force training data. In general, one can also have virial training data.\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.xyz``/``test.xyz`` 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.xyz``/``test.xyz``. There are quite a few tools (https://github.com/brucefan1983/GPUMD/tree/master/tools/nep_related) available for this purpose."
+    "- We used the VASP code to calculate the training data, but ``nep`` does not care about this. You can generate the data in ``train.xyz``/``test.xyz`` 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.xyz``/``test.xyz``. There are quite a few tools (https://github.com/brucefan1983/GPUMD/tree/master/tools) available for this purpose."
    ]
   },
   {