Update M49.cpp

Fix M49 so that it does not hang the terminal console.

Marlin/src/gcode/bedlevel/ubl/M49.cpp

 
 void GcodeSuite::M49() {
   g26_debug_flag ^= true;
-  SERIAL_PROTOCOLPGM("G26 Debug ");
-  serialprintPGM(g26_debug_flag ? PSTR("on.") : PSTR("off."));
+  SERIAL_PROTOCOLPGM("G26 Debug: ");
+  serialprintPGM(g26_debug_flag ? PSTR("On") : PSTR("Off"));
+  SERIAL_EOL();
 }
 
 #endif // G26_MESH_VALIDATION