Fix copy&paste error in diagnostic message.

tests/pio_write.c

@@ -288,7 +288,7 @@ main(int argc, char *argv[])
           setup.nvars = parse_intarg("error parsing number of variables");
           if (setup.nvars < 1)
             {
-              fputs("number of levels must be greater than zero!\n", stderr);
+              fputs("number of variables must be greater than zero!\n", stderr);
               exit(EXIT_FAILURE);
             }
           if (setup.nvars > 127)