fix(guide): Fix incorrect code in Java Strings

[Manish-Giri]

Incorrect Code

String str = "Hello world";
String str2 = "Hello world";

System.out.println(str == str2); // This prints false
System.out.println(str.equals(str2); // This prints true

Using == to compare String objects checks references to the two objects, and in this case, those two references point to the same string object created in the String Pool. Hence this returns true, and not false, as is incorrectly stated in the guide.

Also fixed formatting.

• I have read freeCodeCamp's contribution guidelines.
• My pull request has a descriptive title (not a vague title like Update index.md)
• My pull request targets the master branch of freeCodeCamp.
• None of my changes are plagiarized from another source without proper attribution.
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