bug fix for listing migrating monsters

A typo in the code added to #migratemons resulted in bad output when
listing a subset of migrating monsters if there were any aimed at the
next level.  Didn't affect listing 'a'll because the incorrect code
wasn't reached in that situation.

src/cmd.c

@@ -4087,7 +4087,7 @@ list_migrating_mons(
                     showit = (c == 'c');
                 else if (mtmp->mux == nextlevl->dnum
                          && mtmp->muy == nextlevl->dlevel)
-                    showit = (c = 'n');
+                    showit = (c == 'n');
                 else
                     showit = (c == 'o');