perf help: Change 'usage' to 'Usage' for consistency

perf help: Change 'usage' to 'Usage' for consistency

Capitalize 'usage' to make it consistent with all the other 'Usage' in
the codes, e.g., usage_builtin.

Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ramkumar Ramachandra <artagnon@gmail.com>
Cc: Sriram Raghunathan <sriram.r@nokia.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1444894792-2338-3-git-send-email-yunlong.song@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>