added oop course to sinhala free courses (#8196)

* added oop course to sinhala free courses

* updated oop course file sinhala

* use java category

Co-authored-by: Eric Hellman <eric@hellman.net>

courses/free-courses-si.md

@@ -19,6 +19,7 @@
 
 ### Java
 
+* [Object Oriented Programming ](https://youtube.com/playlist?list=PLqeCu_1ZdDl63h6YR3QsxcGOB7yDS7i3b) - LankaDroid Programming Kuppiya (YouTube)
 * [Sinhala Java Netbeans Lessons](https://youtube.com/playlist?list=PLA3ZeQncjeVu9VHevp2SmPCQ9muVO3fEB) - Chanux Bro (YouTube)