Fix a error of 'book/deref-coercions.html'

[loggerhead]

The original sentence is:

This example has two conversions: Rc<String> to String and then String to &str.

But it should be

This example has two conversions: Rc<String> to String and then String to str.

or

This example has two conversions: &Rc<String> to &String and then &String to &str.

I think the latter is more clearly.

r? @steveklabnik

[rust-highfive]

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[steveklabnik]

@bors: r+ rollup

So sorry about the delay here! Thank you, I agree this is more accurate.

[bors]

📌 Commit 0d3bdc6 has been approved by steveklabnik