perf tools: Fix perf.data documentation units for memory size

The perf.data-file-format documentation incorrectly says the
HEADER_TOTAL_MEM results are in bytes.  The results are in kilobytes
(perf reads the value from /proc/meminfo)

Signed-off-by: Vince Weaver <vincent.weaver@maine.edu>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/alpine.DEB.2.21.1907251155500.22624@macbook-air
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>