libperf: Fix man page typos

libperf: Fix man page typos

Fix various typos and inconsistent capitalization of CPU in the libperf
man pages.

Signed-off-by: Rob Herring <robh@kernel.org>
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/20200807193241.3904545-1-robh@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>