perf docs: Fix a typo in 'perf probe' man page: l20th -> 120th

perf docs: Fix a typo in 'perf probe' man page: l20th -> 120th

Fix a minor typo in 'perf probe' doc.

Fixes: 631c9def804b2c92 ("perf probe: Support --line option to show probable source-code lines")
Signed-off-by: qinyu <qinyu32@huawei.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Feilong Lin <linfeilong@huawei.com>
Cc: Hewenliang <hewenliang4@huawei.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Masami Hiramatsu <mhiramat@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230116012143.432435-1-qinyu32@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>