debugfs: Correct the 'debugfs_create_str' docs

debugfs: Correct the 'debugfs_create_str' docs

The documentation of the 'debugfs_create_str' says that the function
returns a pointer to a dentry created, or an ERR_PTR in case of error.
Actually, this is not true: this function doesn't return anything at all.
Correct the documentation correspondingly.

Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Link: https://lore.kernel.org/r/20230514172353.52878-1-ivan.orlov0322@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>