docs: Update requirements in readme

I discovered today that passing in designated requirements as a string did not work - whereas the exact same string in a text file did indeed work. This PR fixes that in our README.

Works:

osxSign: {
  identity: 'Developer ID Application: Felix Rieseberg (LT94ZKYDCJ)',
  optionsForFile: (filePath) => {
    entitlements: 'static/entitlements.plist',
    requirements: path.resolve(__dirname, 'tools/certs/requirements.txt'),
  },
}

Does not work:

osxSign: {
  identity: 'Developer ID Application: Felix Rieseberg (LT94ZKYDCJ)',
  optionsForFile: (filePath) => {
    entitlements: 'static/entitlements.plist',
    requirements: 'designated => anchor apple generic and certificate 1[field.1.2.840.113635.100.6.2.6] /* exists */ and certificate leaf[field.1.2.840.113635.100.6.1.13] /* exists */ and ( certificate leaf[subject.OU] = LT94ZKYDCJ or certificate leaf[subject.OU] = UY52UFTVTM )'
  },
}

electron / osx-sign

docs: Update requirements in readme #300