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

tools/perf/Documentation/perf-sched.txt

@@ -104,8 +104,8 @@ OPTIONS for 'perf sched timehist'
     kallsyms pathname
 
 -g::
---no-call-graph::
-	Do not display call chains if present.
+--call-graph::
+	Display call chains if present (default on).
 
 --max-stack::
 	Maximum number of functions to display in backtrace, default 5.