Fix syntax error in comprehension example
Fix syntax error in comprehension example

Apparently we missed one instance of the hold comma-based syntax when we
moved over to semicolons.

tsandall/opa,tsandall/opa,open-policy-agent/opa,tsandall/opa,open-policy-agent/opa,tsandall/opa,open-policy-agent/opa,open-policy-agent/opa,tsandall/opa,tsandall/opa,open-policy-agent/opa,open-policy-agent/opa