fix: type declaration for IncomingPhoneNumberInstance

[thinkingserious]

Fixes #529

Checklist

• I acknowledge that all my contributions will be made under the project's license

[thinkingserious]

Tested with:

const accountSid = process.env.TWILIO_ACCOUNT_SID;
const authToken = process.env.TWILIO_AUTH_TOKEN;
const client = require('./lib')(accountSid, authToken);

client.incomingPhoneNumbers('PNXXXXXXXXXXXXXXXXXXXXXXXXXXX')
    .fetch()
    .then(incoming_phone_number => console.log(incoming_phone_number.capabilities));

Response:

{ voice: true, sms: true, mms: true, fax: true }

[childish-sambino]

@thinkingserious Was that a test using TypeScript?

[thinkingserious]

That code was first put into a file temp.ts and compiled with tsc temp.ts which produced temp.js. After executing node temp.js, the above output was received.