BAEL-1090 Fixed minor issue with total time calculation (#2589)

core-java-9/src/main/java/com/baeldung/java9/compactstring/CompactStringDemo.java

@@ -17,7 +17,7 @@ public class CompactStringDemo {
         startTime = System.currentTimeMillis();
         String appended = (String) strings.stream().limit(100_000)
           .reduce("", (left, right) -> left.toString() + right.toString());
-        totalTime = System.currentTimeMillis() - totalTime;
+        totalTime = System.currentTimeMillis() - startTime;
         System.out.println("Created string of length " + appended.length() 
           + " in " + totalTime + " ms.");
     }