Fixed multiple duplicate codepen embed scripts issue

[ishankbahl]

Fix issue #62 by removing the two duplicate CodePen embed scripts on the Thinking in React doc page.

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[bvaughn]

The changes to content/docs/thinking-in-react.md look good. Please revert the modified yarn.lock file though 😄

[bvaughn]

Oh, whoops! Looks like a PR (#63) was already opened before this one to fix this. I'll go ahead and merge the earlier one. Sorry! 😄