AP_Landing: unify singleton naming to _singleton and get_singleton()

libraries/AP_Landing/AP_Landing_Slope.cpp

@@ -107,7 +107,7 @@ bool AP_Landing::type_slope_verify_land(const Location &prev_WP_loc, Location &n
             
             // Check if the landing gear was deployed before landing
             // If not - go around
-            AP_LandingGear *LG_inst = AP_LandingGear::instance();
+            AP_LandingGear *LG_inst = AP_LandingGear::get_singleton();
             if (LG_inst != nullptr && !LG_inst->check_before_land()) {
                 type_slope_request_go_around();
                 gcs().send_text(MAV_SEVERITY_CRITICAL, "Landing gear was not deployed");