when we exec 'obs_serverstatus' command without other pargs,
the return usage message is 'Usage: bs_serverstatus [--top] <statusfile>'.
However, the 'bs_serverstatus' is different with input 'obs_serverstatus'.
Here, we change the usage info to 'Usage: obs_serverstatus [--top] <statusfile>'.
Signed-off-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>
