Fix a typo in my previous commit

Somehow in the late stages of testing my sched_ule patch, a character was
accidentally deleted from the file. Correct this.

While I'm committing anyway, the previous commit message requires some
clarification: in the normal case of unlending priority after releasing
a mutex, the thread that was doing the lending will be woken up and
immediately become the highest-priority thread, and in that case no
priority inversion would take place. However, if that thread is pinned
to a different CPU, then the currently running thread that just had its
priority lowered will not be preempted and then priority inversion can
occur.

Reported by: O. Hartmann (typo), jhb (scheduler clarification)
MFC after: 1 month
Pointy hat to: rstone