Correct WrappedInstance example in React Apollo docs

[shanonvl]

• Resolve logic issue ( component.saySomething() should be component.getWrappedInstance().saySomething() )
• Introduce wrappedInstance constant for clarity

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]

@shanonvl: 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/

[hwillson]

Thanks @shanonvl!