G34 logic / typo fix (#15938)

Marlin/src/gcode/calibrate/G34_M422.cpp

         if (iteration == 0 || i > 0) do_blocking_move_to_z(z_probe);
 
         // Probe a Z height for each stepper.
-        const float z_probed_height = probe_at_point(z_auto_align_pos[i], raise_after, 0, true);
+        const float z_probed_height = probe_at_point(z_auto_align_pos[iprobe], raise_after, 0, true);
         if (isnan(z_probed_height)) {
           SERIAL_ECHOLNPGM("Probing failed.");
           err_break = true;