The 1892 RLDS typesetter accidentally set the singular “the precept of men” instead of the correct plural, “the precepts of men”. Perhaps he was influenced by the preceding use of the singular precept in 2 Nephi 28:30 (“precept upon precept”). In any event, the 1892 typesetter left unchanged the following occurrence of the plural precepts (“save their precepts shall be given by the power of the Holy Ghost”).
Summary: Maintain in 2 Nephi 28:31 both occurrences of the plural precepts.