scripts: align --context path with user-specified bisection_context.yaml

Update the --context path in both the CLI help and execute_bisection.sh
to refer to the correct default filename 'bisection_context.yaml'

Fixes: 2f7460b0 ("scripts: add execute_bisection.sh for git bisect-based regression detection")
Signed-off-by: Aishwarya TCV <aishwarya.tcv@arm.com>