Fix stupid, stupid, stupid oversight in G29 update

Mea culpa!

Marlin/Marlin_main.cpp

             if (HYPOT2(xProbe, yProbe) > sq(DELTA_PROBEABLE_RADIUS) + 0.1) continue;
           #endif
 
-          float measured_z = probe_pt(xProbe, yProbe, stow_probe_after_each, verbose_level);
+          measured_z = probe_pt(xProbe, yProbe, stow_probe_after_each, verbose_level);
 
           #if ENABLED(AUTO_BED_LEVELING_LINEAR)