#386 Fix var name in cdev_alloc example

The cdev_alloc() example declares the variable as my_dev but assigns the ops field through my_cdev.

This change uses a consistent variable name throughout the example.

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Summary by cubic

Use a consistent variable name in the cdev_alloc() example, renaming my_dev to my_cdev so allocation and ops assignment match. This avoids confusion for readers copying the snippet.