Here the 1892 RLDS edition inserted the preposition unto after the verb show /shew. The 1908 RLDS edition removed this intrusive unto from the RLDS text.
The Book of Mormon text has 167 occurrences of shew /show, of which 148 take an indirect object. In most cases, the preposition for the indirect object is expressed, with unto being the most frequent (129 times) and to quite infrequent (only 3 times). There are also 16 cases where no preposition is expressed, as here in Jacob 2:15 (in accord with the earliest textual sources). Here is an example for each of the three possibilities:
There is considerable variation in some passages, as in these examples from Alma 30 (four with unto and three without a preposition):
All three types occur in the text, but since unto dominates, it is not surprising that the 1892 typesetter accidentally introduced the expected unto here in Jacob 2:15. (For one case where the scribe omitted the preposition unto, see Mormon 9:11.) In each case the critical text will follow the earliest textual sources in determining how the indirect object for shew /show should be expressed.
Summary: Maintain the reading in Jacob 2:15 without the preposition unto after the verb shew /show (“O that he would show you that …”).