Update model_mapping.ipynb

fixed a broken link at line 1256

Author: synandi

site/en/guide/migrate/model_mapping.ipynb

@@ -1253,7 +1253,7 @@
         "\n",
         "The above migration process to native TF2 APIs changed both the variable names (as Keras APIs produce very different weight names), and the object-oriented paths that point to different weights in the model. The impact of these changes is that they will have broken both any existing TF1-style name-based checkpoints or TF2-style object-oriented checkpoints.\n",
         "\n",
-        "However, in some cases, you might be able to take your original name-based checkpoint and find a mapping of the variables to their new names with approaches like the one detailed in the [Reusing TF1.x checkpoints guide](./reusing_checkpoints.ipynb).\n",
+        "However, in some cases, you might be able to take your original name-based checkpoint and find a mapping of the variables to their new names with approaches like the one detailed in the [Reusing TF1.x checkpoints guide](https://github.com/tensorflow/docs/blob/master/site/en/guide/migrate/migrating_checkpoints.ipynb).\n",
         "\n",
         "Some tips to making this feasible are as follows:\n",
         "- Variables still all have a `name` argument you can set.\n",