Clarify instructions for bird-watcher, exercise 3

[jrpilat]

The description for Problem 3 of the Bird Watcher go exercise states,

You realized that all the time you were trying to keep track of the birds, there was one bird that was hiding in a far corner of the garden.

You figured out that this bird always spent every second day in your garden.

The the Pearson Education dictionary of contemporary English's defines "every second" to mean:

the second, then the fourth, then the sixth...

Thus, applied to the problem, "every second day" means that the 1st day the bird was not in the garden, the 2nd day it was, the 3rd day it was not, the 4th day it was, and so on and so forth.

bird_watcher_test, however, is coded to test something different: It expects that the 1st day, the bird was in the garden, the 2nd day it was not, the 3rd day it was, the 4th day it was not.

Other dictionaries either confirm this, or point out that the wording currently provided is ambiguous, either of which is undesirable in an exercise such as this. @IsaacG was kind enough to provide an example of a conflicting dictionary entry pointing out the ambiguity of the current wording here. Thank you, Isaac!

Thus, this PR disambiguates the instructions such that everybody is happy :)

[github-actions]

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