Fix M48 output (#20713)

Marlin/src/gcode/calibrate/M48.cpp

         SERIAL_ECHO(n + 1);
         SERIAL_ECHOPAIR(" of ", int(n_samples));
         SERIAL_ECHOPAIR_F(": z: ", pz, 3);
+        SERIAL_CHAR(' ');
         dev_report(verbose_level > 2, mean, sigma, min, max);
         SERIAL_EOL();
       }