perf_event_open.2: Fix wording in multiplexing description

Back in 2014 (37bee118ad692e120559c71bc2b2428d31637c5e) the text
describing when multiplexing happens was changed in a confusing way.
This is an attempt to clarify things a bit.

Signed-off-by: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
1 file changed