perf: Rename perf-examples.txt to examples.txt

Rename it to examples.txt to avoid the perf-*.txt pattern in
the Makefile, otherwise 'make doc' fails because
perf-examples.txt is not formatted to be a man page:

 ERROR: perf-examples.txt: line 1: manpage document title is mandatory
Signed-off-by: NCarlos R. Mafra <crmafra@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <new-submission>
Signed-off-by: NIngo Molnar <mingo@elte.hu>