zswap: docs/vm: Fix typo accept_threshold_percent in zswap.rst

zswap: docs/vm: Fix typo accept_threshold_percent in zswap.rst

Recently, I switched over from swap-file to zramswap.

When reading the Documentation/vm/zswap.rst file I fell over this typo.

The parameter is called accept_threshold_percent not accept_threhsold_percent
in /sys/module/zswap/parameters/ directory.

Fixes: 45190f01dd402 ("mm/zswap.c: add allocation hysteresis if pool limit is hit")
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Signed-off-by: Sedat Dilek <sedat.dilek@gmail.com>
Link: https://lore.kernel.org/r/20200601005911.31222-1-sedat.dilek@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>