This change from saith to said undoubtedly represents a typo by the typesetter for the 1911 LDS edition. This past-tense said has continued in the LDS text. The surrounding text, however, uses saith rather than said:
The general expression “saith God” is correct throughout this chapter since the text continually refers to various promises from the Lord that represent his eternally lasting word. Prophetic statements are normally given in the present tense, as in “thus saith the Lord God”.
Summary: Restore in 2 Nephi 10:10 the present-tense saith, the reading of the original text; the text here refers to a promise of the Lord’s that holds throughout time.