perf tools: Fix perf.data format description of NRCPUS header

In the perf.data HEADER_CPUDESC feadure header we store first the number
of available CPUs in the system, then the number of CPUs at the time of
writing the header, not the other way around.

Reported-by: Thomas-Mich Richter <[e-mail]>
Acked-by: Andi Kleen <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: David Ahern <[e-mail]>
Cc: He Kuang <[e-mail]>
Cc: Hendrik Brueckner <[e-mail]>
Cc: Jin Yao <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Kim Phillips <[e-mail]>
Cc: Lakshman Annadorai <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Simon Que <[e-mail]>
Cc: Stephane Eranian <[e-mail]>
Cc: Wang Nan <[e-mail]>
Link: https://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

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

···
 A structure defining the number of CPUs.
 
 struct nr_cpus {
-       uint32_t nr_cpus_online;
        uint32_t nr_cpus_available; /* CPUs not yet onlined */
+       uint32_t nr_cpus_online;
 };
 
 	HEADER_CPUDESC = 8,