tour: updated description for `if` statement in flow control article

[sagar23sj]

Existing article is misleading, in new change clear description is provided.

Fixes: golang/tour#743

[gopherbot]

This PR (HEAD: fe495cd2e9b615917d254cc2b00b1deff60c5d36) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/website/+/328251 to see it.

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