fs/char_dev.c: fix incorrect documentation for unregister_chrdev_region

The current documentation for unregister_chrdev_region says that it return
a range of device numbers which is incorrect.  Instead it unregister a
range of device numbers.  Fix the documentation to make this clear.

Signed-off-by: Partha Pratim Mukherjee <ppm.floss@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>