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: NVince 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-airSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf.data-file-format.txt

@@ -126,7 +126,7 @@ vendor,family,model,stepping. For example: GenuineIntel,6,69,1
 
 	HEADER_TOTAL_MEM = 10,
 
-An uint64_t with the total memory in bytes.
+An uint64_t with the total memory in kilobytes.
 
 	HEADER_CMDLINE = 11,