Fix mockServer example

[dougshamoo]

TODO:

• If this PR is a new feature, 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
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change. Include a description of your change, link to PR (always) and issue (if applicable). Add your CHANGELOG entry under vNEXT. Do not create a new version number for your change yourself.

For reference, the mockServer function signature: https://github.com/apollographql/graphql-tools/blob/b85137ba81c9bdc5667bced9cad9b1d226ea83fc/src/mock.ts#L34-L38

I recently set up graphql mocking for the test environment in one of our apps, and it took me way longer than I care to admit to figure out that the single doc describing how to use mockServer was incorrect (should have looked at the source code a lot earlier in retrospect). This change fixes the example and adds a little more info to help prevent people from going down the wrong path.

All that being said, in the future I think I would prefer to see the mockServer function take an object of named parameters like many of the other related functions (notably makeExecutableSchema and addMockFunctionsToSchema which are described in the same doc and take confusingly similar arguments). So, I wouldn't be upset if this PR turned into that.

Thanks for all the hard work.

[apollo-cla]

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