The 1849 LDS edition accidentally replaced “this proclamation” with “his proclamation”, probably because of the visual similarity between this and his coupled with the following use of his in “unto his people”. The LDS text has retained this typo. In the following verse, the text has one more instance of “this proclamation”, one that has never been changed to “his proclamation”:
Elsewhere the text has only instances of “a proclamation” (12 times) and “the proclamation” (3 times), but never “his proclamation”.
Summary: Restore in Alma 23:3 the determiner this in “he sent this proclamation”, the reading of the original manuscript.