Correct description of BN_mask_bits
Correct description of BN_mask_bits

CLA: trivial

Correct right shift to left shift.
Pseudo code `a&=~((~0)>>n)` means "get higher n-bits of a", but actually crypto lib gives lower n-bits.

Reviewed-by: Matt Caswell <1fa2ef4755a9226cb9a0a4840bd89b158ac71391@openssl.org>
Reviewed-by: Paul Dale <bc4a6b4cd7a8395c734c8eb5aef04a9c30188c85@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/12727)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl