Here the 1874 RLDS edition accidentally replaced this in “unto this people” with his, probably because of the his earlier in this sentence (“by his Holy Spirit”). Theoretically, either reading will work, although it does seem a little strange for Ammon to refer to these Lamanites as the Lord’s people prior to their conversion. The critical text will, of course, maintain the original this in this passage. The 1908 RLDS edition restored the this to the RLDS text.
Summary: Maintain in Alma 18:34 the original this in “I am called by his Holy Spirit to teach these things unto this people”.