docs: makes Courier http doc consistent with kratos configuration

[adamstrawson]

The documentation is inconsistent with the format Kratos configuration expects, this PR brings them inline.

1. headers is plural in the schema

 "headers": {
          "type": "object",
          "description": "The HTTP headers that must be applied to request",
          "additionalProperties": {
            "type": "string"
          }
        },

2. email.strategy was later replaced with delivery_strategy

Related Issue or Design Document

ory/kratos#3392

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[kmherrmann]

thank you!