perf doc: Fix typo in perf.data-file-format.txt

perf doc: Fix typo in perf.data-file-format.txt

The 'it' should be 'is' here, fix it.

Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230727105001.261420-1-xiujianfeng@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>