Here Oliver Cowdery initially wrote “the words which I desired”, then corrected it to “the words that I desired”. His correction was virtually immediate since the level of ink flow is unchanged. At the beginning of this verse, there is an occurrence of “the words which”:
This preceding instance is undoubtedly the source for Oliver initially writing that same phrase later on in the verse. Elsewhere in the Book of Mormon text, there are 132 occurrences of “the words which” and 4 of “the words that” (2 Nephi 27:24, Mosiah 29:4, Alma 12:2, and Alma 15:7). So “the words that” is possible, although infrequent. For further discussion of the variation between the relative pronouns which and that, see under which in volume 3.
Summary: Accept in Mosiah 5:6 Oliver Cowdery’s virtually immediate correction in 𝓟, “the words that I desired”.