tour: Reader has method not func

[crisman]

Interfaces do not define func as in the current text of "Readers" ("/tour/methods/21"). The interface specifies a list of methods, just Read() in this case. It is confusing to say "interface has a Read method" and then show a code line defining what would be a method on a type that impliments the interface.

Changed the article text to follow the pattern of "Stringers" ("/tour/methods/17") which shows the interface definition in the code block. In following this pattern I moved the code just below that text about the package io and its io.Reader interface.

Fixes golang/tour#1450

Change is in two commits to try to avoid stomping on the blame log for the line between "The Go standard library contains...". Not sure this was successful, useful, or needed.

[gopherbot]

This PR (HEAD: 93f28464fa2c9ed1e68d61f6711d0e6302498916) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/website/+/471176 to see it.

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