Doc: libata: Fix spelling typo found in libata.xml

This patch fix a spelling typo found in libata.xml.
It is because libata.xml is generated from comments
in source, I have to fix it in libata-core.c

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>