Documentation: zram: fix the description about orig_data_size of mm_stat

orig_data_size counted the same_pages by commit 51f9f82c ("zram:
count same page write as page_stored"), so let's fix it.

Signed-off-by: Yue Hu <zbestahu@163.com>
Link: https://lore.kernel.org/r/20200206111031.9524-1-zbestahu@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>