Update ConfigurationStore.cpp

Done

Marlin/ConfigurationStore.cpp

 #endif//FWRETRACT
 #ifdef CUSTOM_M_CODES
   SERIAL_ECHO_START;
-  SERIAL_ECHOLNPGM("Z-Probe Offset");
+  SERIAL_ECHOLNPGM("Z-Probe Offset (mm):");
   SERIAL_ECHO_START;
   SERIAL_ECHO("   M");
   SERIAL_ECHO(CUSTOM_M_CODE_SET_Z_PROBE_OFFSET);