Update server-side-rendering.md

[jtassin]

In my opinion, this line will be more understandable with the JSON.stringify.

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

[apollo-cla]

@jtassin: 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/

[danilobuerger]

Thanks @jtassin