Documentation: fix sysfs-ptp

s/avaiable/available/g

This fixup is already in scripts/spelling.txt.

The fix in Documentation/ABI/testing/sysfs-ptp affects documentation of
a /sys entry: the /sys entry itself is correct.

Signed-off-by: Chris Dunlop <chris@onthe.net.au>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>