tour: let reader know Walk should send in order

[crisman]

In "Exercise: Equivalent Binary Trees" neither the slide text nor the code state Walk() should send the values in order. Doing so in order makes using the output much more useful in Same(), but might not be easy to see for readers that have not had a data structures class. In order traversal is implied by reference with the binary tree being "always sorted" and the output defined as "should be the numbers 1, 2, 3, ..., 10."

Update the slide text to say what Walk() does. Change Walk() comments in code/solution to match.

Fixes golang/tour#158

[gopherbot]

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