perf tools: Change default calchain percent limit to 0.005%

The current default limit of 0.5% is (mostly) for 'fractal' mode which
calculates the percentage relatively.  As we changed the default
callchain mode to 'graph', the existing limit is too high IMHO.
Normally there're many entries under 0.5% overhead, it'd be supprising
that they don't show callchains.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>