perf doc: Fix documentation of the Flags section in perf.data

perf doc: Fix documentation of the Flags section in perf.data

According to the current documentation the flags section is placed after
the file header itself but the code assumes to find the flags section
after the data section. This change updates the documentation to that
assumption.

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/20190219154515.3954-2-jonas.rabenstein@studium.uni-erlangen.de
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>