Remove improper '1' from doctest

The doctest didn’t match the README and spec and was [changed on
exercism/xelixir][ee].

Thanks to @lancehalvorsen for [pointing this out][s]!

[ee]: https://github.com/exercism/xelixir/commit/1810a3dcf0f702605cf8cc1b5d00ff91e7189905
[s]: http://exercism.io/submissions/e1a12f380085153d0c9b10a1
Limit calls to number function

Per a [suggestion][s] from @lancehalvorsen.

[s]: http://exercism.io/submissions/e1a12f380085153d0c9b10a1

Submission http://exercism.io/submissions/e5a0f6e9c1f31162f5c49da3