Fix format of license field in 'package.json'

[nfreear]

Hi @websanova,

Thanks for developing vue-auth.

QUESTION :~ Is this module dual or single licensed please?

LICENSE and README only mention MIT, while package.json says "MIT, GPL".

If it is dual-licensed, then the correct format in package.json is something like:

{
    "license" : "(MIT OR GPL-3.0)"
}

See:

• https://docs.npmjs.com/cli/v7/configuring-npm/package-json#license;
• https://npmjs.com/package/spdx;

Thank you!

Nick