sched/Documentation/sched-rt-group: Fix incorrect example

sched/Documentation/sched-rt-group: Fix incorrect example

I feel that the example given in the document to show the possibility
of task starvation of configurable period is wrong.

The example says group A and B both have 50% bandwidth, and a while (1)
loop in A will run for the full period of B and can starve B's tasks.

So I think the runtime of group A should be 50000us, then the period and
runtime of group B should be 50000us and 25000us.

Signed-off-by: Zhou Chengming <zhouchengming1@huawei.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: corbet@lwn.net
Cc: iamyooon@gmail.com
Cc: linux-doc@vger.kernel.org
Cc: lizefan@huawei.com
Link: http://lkml.kernel.org/r/1485069755-44287-1-git-send-email-zhouchengming1@huawei.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>