fix: removed trailing slash

Author: LuigiImVector

books/free-programming-books-zh.md

@@ -322,7 +322,7 @@
 ### C
 
 * [新概念 C 语言教程](https://github.com/limingth/NCCL)
-* [Beej's Guide to Network Programming 簡體中文版](https://beej-zhtw-gitbook.netdpi.net/) - Brian "Beej Jorgensen" Hall, 廖亚伦译
+* [Beej's Guide to Network Programming 簡體中文版](https://beej-zhtw-gitbook.netdpi.net) - Brian "Beej Jorgensen" Hall, 廖亚伦译
 * [C 语言常见问题集](http://c-faq-chn.sourceforge.net/ccfaq/ccfaq.html)
 * [Linux C 编程一站式学习](https://web.archive.org/web/20210514225440/http://docs.linuxtone.org/ebooks/C&CPP/c/) *(:card_file_box: archived)*