The original manuscript is not extant here, but Oliver Cowdery’s erasure in 𝓟 of the final s indicates an immediate change, probably to the original reading. The preceding singular noun cry supports the singular reading prayer, especially in light of the consistent number agreement between cry/cries and prayer /prayers elsewhere in the text (although all these other examples are in the plural):
One could argue that given the singular cry in Jacob 7:22, we expect the matching singular prayer. In any event, the singular prayer is the reading of the earliest textual sources.
Summary: Retain in Jacob 7:22 the singular prayer, Oliver Cowdery’s correction in 𝓟 to the probable reading in 𝓞.