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 <[email protected]>
Cc: Hemant Kumar <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Srikar Dronamraju <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>