perf Documentation: Correct branch stack sampling call-stack option

The correct call-stack option for branch stack sampling should be "stack"
instead of "call_stack". Correct it.

$perf record -e instructions -j call_stack -- sleep 1
unknown branch filter call_stack, check man page

 Usage: perf record [<options>] [<command>]
    or: perf record [<options>] -- <command> [<options>]

    -j, --branch-filter <branch filter mask>
                          branch stack filter modes

Fixes: 955f6def ("perf record: Add remaining branch filters: "no_cycles", "no_flags" & "hw_index"")
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Falcon <thomas.falcon@intel.com>
Cc: Xudong Hao <xudong.hao@intel.com>
Cc: Zide Chen <zide.chen@intel.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>