[ZainnQureshii]

What does this PR do?

docs/source-pytorch/debug/debugging_basic.rst:48 says Trainer(fast_dev_run=True) "runs 5 batch of training, validation, test and prediction data". It runs one: _init_debugging_flags sets the batch limits to num_batches = int(fast_dev_run) (src/lightning/pytorch/trainer/setup.py:61-68), and int(True) is 1. The Trainer reference already says "1 if set to True" (docs/source-pytorch/common/trainer.rst:559).

This PR changes "5 batch" to "1 batch". The integer example below it (fast_dev_run=7) is already right.

Counted the training_step and validation_step calls of a BoringModel under Trainer(fast_dev_run=..., accelerator="cpu").fit(...) with lightning 2.6.6:

fast_dev_run=True -> {'train': 1, 'val': 1}
fast_dev_run=7 -> {'train': 7, 'val': 7}

The docs build was not run locally.

Docs only, so no linked issue, tests or CHANGELOG entry.

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