perf probe: Trivial typo fix for --demangle

perf probe: Trivial typo fix for --demangle

Replace "Disable" with "Enable", since --demangle option enables symbol
demangling, not disable it.

perf probe has --demangle and --no-demangle options, but the
command-line help (--help) shows only --demangle option. So it should
explain about --demangle.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Hemant Kumar <hemant@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20141027203124.21219.68278.stgit@localhost.localdomain
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>