Fix incorrect print stat in ExtUI (#14881)

Marlin/src/lcd/extensible_ui/ui_api.cpp

@@ -792,7 +792,7 @@ namespace ExtUI {
     char* getTotalPrints_str(char buffer[21])    { strcpy(buffer,i16tostr3left(print_job_timer.getStats().totalPrints));    return buffer; }
     char* getFinishedPrints_str(char buffer[21]) { strcpy(buffer,i16tostr3left(print_job_timer.getStats().finishedPrints)); return buffer; }
     char* getTotalPrintTime_str(char buffer[21]) { duration_t(print_job_timer.getStats().printTime).toString(buffer);       return buffer; }
-    char* getLongestPrint_str(char buffer[21])   { duration_t(print_job_timer.getStats().printTime).toString(buffer);       return buffer; }
+    char* getLongestPrint_str(char buffer[21])   { duration_t(print_job_timer.getStats().longestPrint).toString(buffer);    return buffer; }
     char* getFilamentUsed_str(char buffer[21])   {
       printStatistics stats = print_job_timer.getStats();
       sprintf_P(buffer, PSTR("%ld.%im"), long(stats.filamentUsed / 1000), int16_t(stats.filamentUsed / 100) % 10);