fix typo in docs migration 1_6_regular (#17186)

(cherry picked from commit 7ec9a97)

Author: gkroiz

docs/source-pytorch/upgrade/sections/1_6_regular.rst

@@ -50,7 +50,7 @@
      - use the utility function ``pl.utilities.memory.get_model_size_mb(model)``
      - `PR8495`_
 
-   * - relied on the ``on_trai_dataloader()`` hooks in  ``LightningModule`` and ``LightningDataModule``
+   * - relied on the ``on_train_dataloader()`` hooks in  ``LightningModule`` and ``LightningDataModule``
      - use ``train_dataloader``
      - `PR9098`_