[Docs][llvm-exegesis] Add documentation for --use-dummy-perf-counters

[aidengrossman]

Summary

This patch adds in documentation for the --use-dummy-perf-counters
option (introduced in D146301).

[kpdev42]

llvm/docs/CommandGuide/llvm-exegesis.rst

365

Small typo dumm>p<y

[kpdev42]

@aidengrossman thank you. LGTM (only need to fix typo)

llvm/docs/CommandGuide/llvm-exegesis.rst

.. option:: --analysis-override-benchmark-triple-and-cpu
   were measured for, but if you want to analyze them for some other combination
   (specified via `-mtriple`/`-mcpu`), you can pass this flag.
 
 .. option:: --dump-object-to-disk=true
 
  If set,  llvm-exegesis will dump the generated code to a temporary file to
  enable code inspection. Disabled by default.
 
+.. option:: --use-dummy-perf-counters
+
+ If set, llvm-exegesis will not read any real performance counters and
+ return a dummy value instead. This can be used to ensure a snippet doesn't
+ crash when hardware performance counters are unavailable and for
+ debugging :program:`llvm-exegesis` itself.
+
 EXIT STATUS
 -----------
 
 :program:`llvm-exegesis` returns 0 on success. Otherwise, an error message is
 printed to standard error, and the tool returns a non 0 value.