perf doc: Fix HEADER_CMDLINE description in perf.data documentation

perf doc: Fix HEADER_CMDLINE description in perf.data documentation

The content of the HEADER_CMDLINE feature header is a perf_header_string_list
of the argument vector and not a perf_header_string of the commandline.

Signed-off-by: Jonas Rabenstein <jonas.rabenstein@studium.uni-erlangen.de>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Link: http://lkml.kernel.org/r/20190219154515.3954-1-jonas.rabenstein@studium.uni-erlangen.de
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>