Here we have an obvious typo in the 1907 LDS vest-pocket edition, perhaps caused by the sh in the immediately preceding shall that the typesetter would have just set (thus “shall show”). Note that the error was not made in the next verse, which reads “shall sow” in all the (extant) textual sources, including the 1907 edition. The typo in verse 30 is, of course, not totally impossible; the people could show (or exhibit) filthiness, although show is clearly unexpected in the larger passage that refers to sowing and reaping.
Summary: Maintain the phraseology “if my people shall sow filthiness”, which occurs twice in Mosiah 7:30–31.