PM / Documentation: Fix minor issue in freezing_of_tasks.txt

In a paragraph, "kernel thread" is mistakenly written as "kernel". Fix this by
adding thread after word "kernel".

Changes are shown in multiple lines, as they are realigned to 80 col width.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>