doc: fix some typos in documentations

Fix some typos in five documentations, no functional change.

Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

diff --cc Documentation/md.txt
Simple merge

diff --cc Documentation/rfkill.txt
Simple merge

diff --cc Documentation/rt-mutex-design.txt
Simple merge

diff --cc Documentation/static-keys.txt
Simple merge