pid: Fix spelling in comments

Accidentally discovered this typo when I studied this module.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Cc: Hanjun Guo <guohanjun@huawei.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tianhong Ding <dingtianhong@huawei.com>
Cc: Xinwei Hu <huxinwei@huawei.com>
Cc: Zefan Li <lizefan@huawei.com>
Link: http://lkml.kernel.org/r/1454119457-11272-1-git-send-email-thunder.leizhen@huawei.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>