ouch201: Fix IssuePriceStabilizing and RisklessPrincipal constants

According to the OUCH 2.01 specification section 3.1, value 6 stands for
"Issue price stabilizing" capacity and 7 for "Riskless Principal".

Signed-off-by: Ville Aine <ville.aine@valotrading.com>
Merge pull request #162 from vaine/fixes/ouch201-capacity

ouch201: Fix IssuePriceStabilizing and RisklessPrincipal constants