Update ExchangeRate connector apiDefinition.swagger.json with missing "produces" property

[yeze322]

When testing with random connectors in PVA, we found this connector cannot be parsed correctly by PFX parser.
This PR fixes this issue by providing "produces" field in the Swagger json.

We've tested in PowerFx source repository that this fix can make it recognized, and no functional changes introduced.

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[vmanoharas]

[[certify-connector]]

[vmanoharas]

Hello @yeze322
I hope you are doing well.
Congratulations, your pull request is approved. We will move forward with the certification process. We are preparing your connector for production deployment and will queue up for the next deployment schedule. Once your connector onboards the next deployment schedule, it starts to deploy your connector in our production environments that typically takes 3-4 weeks.
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[vmanoharas]

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