perf docs: Fix accidental em-dashes

perf docs: Fix accidental em-dashes

" -- " is an em dash (—) in asciidoc, so all these examples that were
supposed to be producing a literal two dashes were being misrendered.

Signed-off-by: Alyssa Ross <hi@alyssa.is>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20210809153226.332545-1-hi@alyssa.is
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>