_content/tour/flowcontrol/defer.go: add example for arguments eager evaluation

[Yuhao-C]

This adds an example in the provided code snippet
to demostrate and help user better understand the
eager evaluation of the deferred call's arguments.

Also fixes: golang/tour#1558

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Patch Set 1:

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[gopherbot]

Message from Yuhao Chen:

Patch Set 2:

(1 comment)

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