perf sched: Fix documentation for timehist

Fixed a incorrect option and usage to those shown by "perf sched timehist -h",
i.e. the default is really --call-graph, which is equivalent to -g.

Signed-off-by: Takuya Yamamoto <tkydevel@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/n/tip-8fzo0dlsi1mku5aqx8brep5s@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>