perf annotate TUI: Fix column header when toggling period/percent

perf annotate TUI: Fix column header when toggling period/percent

We have the 't' hotkey to toggle showing either the total period or the
percentage of samples for a given line, but we forgot to toggle as well
the column header, always showing "Percent", even when showing the
period, fix it.

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.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: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1501172169-6761-1-git-send-email-treeze.taeung@gmail.com
[ Extracted from a larger patch, s/Event count/Period/g ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>