bird-watcher: Fix introduction example

[caioguedes]

Small fix in an example that can't be evaluated.

I am not sure if could be better move the example to:

(def three-values [4 5 6])

as the exercise is presented in part of the Numbers path... What do you thing?

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[tasxatzial]

Hello,

Since the preceding paragraph says "heterogenous types", it's probably best if we use a vector that has items of different types.

With this change we'll also be consistent with the article that introduces the concept of vectors:

https://exercism.org/tracks/clojure/concepts/vectors

[IsaacG]

Forum discussion: https://forum.exercism.org/t/bird-watcher-fix-introduction-example/13554/3