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.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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

@@ -43,7 +43,7 @@ struct perf_file_section {
 
 Flags section:
 
-For each of the optional features a perf_file_section it placed after the data
+For each of the optional features a perf_file_section is placed after the data
 section if the feature bit is set in the perf_header flags bitset. The
 respective perf_file_section points to the data of the additional header and
 defines its size.