perf annotate: Document --show-total-period option

When the --show-total-period option was introduced we forgot to add an
entry in the man page, fix it.

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Martin Liška <mliska@suse.cz>
Fixes: 0c4a5bc ("perf annotate: Display total number of samples with --show-total-period")
Link: http://lkml.kernel.org/r/1503046013-5555-1-git-send-email-treeze.taeung@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-annotate.txt

@@ -92,6 +92,8 @@ OPTIONS
 --asm-raw::
 	Show raw instruction encoding of assembly instructions.
 
+--show-total-period:: Show a column with the sum of periods.
+
 --source::
 	Interleave source code with assembly code. Enabled by default,
 	disable with --no-source.