Update java-quiz.md

[KidusMT]

update on the java question #7 answer which was a wrong answer.

jshell> str.substring(3,4);
$1 ==> "d"

[rradfar]

If we try try running this piece of code, it outputs 'abcde'. I believe the reason for this behavior is that the 'toUpperCase()' and 'substring()' methods do not mutate the original string which is what we are printing at the end.

String str = "abcde";
str.trim();
str.toUpperCase();
str.substring(3, 4);
System.out.println(str);  //abcde

If I recall correctly, strings in Java are not mutable so any operation on a string, just returns a new string.

[KidusMT]

Correctly put @rradfar . I only tested it on jshell for that segment of code. I didn't try to "System.out.println" it.. so yes.correct. Discard this pull request.