Update downloading-schema.md

[raudabaugh]

Passing the schema.json argument to Apollo CLI results in this error:

TypeError: Cannot read property 'trim' of undefined
    at Object.parse (~/.nvm/versions/node/v8.11.2/lib/node_modules/apollo/lib/commands/schema/download.js:52:57)
    at Parser._flags (~/.nvm/versions/node/v8.11.2/lib/node_modules/apollo/node_modules/@oclif/parser/lib/parse.js:149:49)
    at Parser.parse (~/.nvm/versions/node/v8.11.2/lib/node_modules/apollo/node_modules/@oclif/parser/lib/parse.js:112:28)
    at Object.parse (~/.nvm/versions/node/v8.11.2/lib/node_modules/apollo/node_modules/@oclif/parser/lib/index.js:25:27)
    at SchemaDownload.parse (~/.nvm/versions/node/v8.11.2/lib/node_modules/apollo/node_modules/@oclif/command/lib/command.js:68:41)
    at SchemaDownload.run (~/.nvm/versions/node/v8.11.2/lib/node_modules/apollo/lib/commands/schema/download.js:13:38)
    at SchemaDownload._run (~/.nvm/versions/node/v8.11.2/lib/node_modules/apollo/node_modules/@oclif/command/lib/command.js:29:31)

[apollo-cla]

@raudabaugh: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[raudabaugh]

relevant: apollographql/rover#528

[Camji55]

@raudabaugh Try closing this request and re-opening to run the CI build again. It shouldn't have failed with your change, and I want to see if the same result occurs.

[Camji55]

@martijnwalraven Are you able to run a manual CI Test on the current repo? This pull request failed, and shouldn't have with the change it made.

[martijnwalraven]

It passed after I restarted it, thanks!