Standardize spelling of "power of two"

Hello.

pg_resetwal and initdb has an error message like this:

msgid "argument of --wal-segsize must be a power of 2 between 1 and 1024"

In other parts in the tree, however, we spell it as "power of two". I
think it would make sense to standardize the spelling for
consistency. See the attached.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center