Update free-courses-bn.md (#11274)

Author: metabiswadeep

courses/free-courses-bn.md

@@ -27,6 +27,7 @@
 * [Linux](#linux)
 * [MongoDB](#mongodb)
 * [MySQL](#mysql)
+* [Operating Systems](#operating-systems)
 * [PHP](#php)
     * [Laravel](#laravel)
 * [Programming paradigms](#programming-paradigms)
@@ -273,6 +274,11 @@
 * [MySQL Database Bangla Tutorials (HSC student / Anyone)](https://www.youtube.com/playlist?list=PLgH5QX0i9K3qLcx9DvVDWmNJ7riPvxzCD) - Anisul Islam
 
 
+### Operating Systems
+
+* [Operating System (OS) Bangla Tutorial (কমপ্লিট কোর্স)](https://www.youtube.com/playlist?list=PLncy2sD7w4Yr3ZbiP_ipAjgjDRn86N_tT) - Lecturelia - CSE Bangla Tutorial
+
+
 ### PHP
 
 * [PHP All You Need To Know](https://www.youtube.com/playlist?list=PL_XxuZqN0xVCFLIrGA1GaxacvPTDQcsMV) - Stack Learner