perf docs: Correct typo of event_sources

perf docs: Correct typo of event_sources

The sysfs directory is called event_source.

Before:

  $ ls -la /sys/bus/event_sources/devices/cpu/format/
  ls: cannot access '/sys/bus/event_sources/devices/cpu/format/': No such file or directory
  $

After:

  $ ls -la /sys/bus/event_source/devices/cpu/format/
  total 0
  drwxr-xr-x. 2 root root    0 Jun  2 15:36 .
  drwxr-xr-x. 6 root root    0 Jun  2 15:35 ..
  -r--r--r--. 1 root root 4096 Jun  2 15:36 any
  -r--r--r--. 1 root root 4096 Jun  2 15:36 cmask
  -r--r--r--. 1 root root 4096 Jun  2 15:36 edge
  -r--r--r--. 1 root root 4096 Jun  2 15:36 event
  -r--r--r--. 1 root root 4096 Jun  2 15:36 frontend
  -r--r--r--. 1 root root 4096 Jun  2 15:36 inv
  -r--r--r--. 1 root root 4096 Jun  2 15:36 ldlat
  -r--r--r--. 1 root root 4096 Jun  2 15:36 offcore_rsp
  -r--r--r--. 1 root root 4096 Jun  2 15:36 pc
  -r--r--r--. 1 root root 4096 Jun  2 15:36 umask
  $

Reviewed-by: Sandipan Das <sandipan.das@amd.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexey Bayduraev <alexey.v.bayduraev@linux.intel.com>
Cc: Alyssa Ross <hi@alyssa.is>
Cc: German Gomez <german.gomez@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Joshua Martinez <joshuamart@google.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Like Xu <likexu@tencent.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Zhengjun Xing <zhengjun.xing@linux.intel.com>
Link: https://lore.kernel.org/r/20220603045744.2815559-1-irogers@google.com
Reported-by: Kevin Nomura <nomurak@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>