Reword user-facing message for "power of two"
While there are numerous instances of using "power of 2" in the code,
translated user-facing messages use "power of two". Fix two instances
which used "power of 2" instead.
Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/
20230829.175615.
682972421946735863.horikyota.ntt@gmail.com