Clarify Oregon EITC age limit is "younger than"

The [OR EITC form](https://www.oregon.gov/dor/forms/FormsPubs/form-or-40-inst_101-040-1_2021.pdf#page=18) says the young child match is for children "younger than 3"

We model it correctly: https://github.com/PolicyEngine/policyengine-us/blob/master/policyengine_us/variables/gov/states/or/tax/income/credits/or_eitc.py#L23

But we document the parameter ambiguously: https://github.com/PolicyEngine/policyengine-us/blob/master/policyengine_us/parameters/gov/states/or/tax/income/credits/eitc/young_child_age.yaml#L1