docs(guide/Expressions): grammar/flow/punctuation

[brianjscoles]

This pull request rewords the introductory sentence of the section "How to benefit from one-time binding" for greater clarity and consistency, and moves it to the top of the section - before, the introductory sentence was mixed in to the first example, which was ambiguous and harder to read.

It also adds colons before each code snippet - there was simply no punctuation before.

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[brianjscoles]

Google CLA has now been signed (github username: brianjscoles).

Thanks!

[googlebot]

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[pkozlowski-opensource]

Thnx!