cli: "result show": fix argparse error from unescaped '%'

argparse interprets '%' as a format specifier, causing 'result show --help' to fail. Escape the literal '%' in the confidence-interval help message.

Fixes: 0fefab68 ("cli: "result show" Add --confidence-interval option")

Signed-off-by: Kelsey Steele kelsey.steele@arm.com