fix: README_flux2.md fsdp config error

[Thundermean-sky]

What does this PR do?

Fix incorrect FSDP configuration examples in the examples/dreambooth/README_flux2.md.

While training, I encountered two issues with the config examples in the README:

1. The value of fsdp_auto_wrap_policy was incorrect, which caused training to fail.
2. There was an extra space between the two class names in fsdp_transformer_layer_cls_to_wrap (Flux2TransformerBlock, Flux2SingleTransformerBlock). Removing the space resolved the error.

After correcting both parameters, the training runs successfully.

This PR updates the README with the corrected FSDP configuration examples.

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[Thundermean-sky]

Note:
Regarding the space between Flux2TransformerBlock and Flux2SingleTransformerBlock in fsdp_transformer_layer_cls_to_wrap: I removed the space because it caused an error in my environment. I'm not sure if this is a general issue or something specific to my setup. Feedback is welcome.

[github-actions]

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