Fix warning message formatting in save_hyperparameters

[awaelchli]

What does this PR do?

Follow up to #12160

The warning message gets rendered like this:

UserWarning: Attribute 'model' is an instance of `nn.Module` and is already saved during checkpointing. 
It is recommended to ignore them using `self.save_hyperparameters(ignore=[model!r])`.

However, it should show:

`self.save_hyperparameters(ignore=['model'])`.

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