Typo correction for description in gpio document.

Corrected Documentation/gpio/sysfs.txt for typos and wording.
typos: syfs -> sysfs, manges -> manages
wording: entry -> entries

Signed-off-by: Sachin Pandhare <sachinpandhare@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>