Parameter "{field_name}" should be renamed to "field_name" in contacts search endpoint

[julealgon]

Issue Summary

The contact recipients search endpoint takes a query string parameter that is currently called {field_name}. This name appears to be invalid and cannot be used as an identifier name in most languages, breaking client proxy generation.

I believe the parameter name should be field_name instead.

https://github.com/sendgrid/sendgrid-oai/blob/072cebebfb3f3139a34727593a9a4241cb4cc80d/spec/paths/legacy_marketing_campaigns/contacts_api_recipients.json#L952