Customer_id example in rule #174 conflicts with backward compatibility constraints

[tfrauenstein]

https://opensource.zalando.com/restful-api-guidelines/#174
gives customer_id as example for good naming which is in principle right. However, here we have to make very clear that there are backward compatibility constraints in Zalando where customer_number and customer_id have been defined with different semantics.

[ePaul]

@tfrauenstein will open a PR.