remove extra '/' from docs

[amitport]

the url is incorrect: example.com//.well-known/apollo/server-health

TODO:

• Update CHANGELOG.md with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

[apollo-cla]

@amitport: 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/

[unicodeveloper]

Hey, @amitport Thanks for the catch!