perf tools: Fix typos / broken sentences

- Fix a typo in the man page
- Fix a tip that doesn't make any sense.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20190628220900.13741-1-andi@firstfloor.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>