Isaiah 5:10 (King James Bible) and the seed of an homer shall yield an ephah
As discussed under 2 Nephi 13:7, the Book of Mormon quotation of Isaiah 2–14 prefers a rather than an before h-initial words, thus a homer instead of the King James an homer.
Here in 2 Nephi 15:10, the 1830 typesetter accidentally misread homer as horner. This error was corrected in the subsequent edition (1837). Just like the word ephah, the word homer is directly taken from the Hebrew and was otherwise unknown to the compositor. Since he did not recognize the word, he simply set what he thought it looked like. Apparently, he did not consult his King James Bible here.
Summary: Maintain the a before homer in 2 Nephi 15:10; the 1830 horner is simply a misreading of the printer’s manuscript.