Update n+1 problem resource (#5557)

* Update 103-n-plus-one-problem.md

Remove link to payed content and provide free access article

* Update src/data/roadmaps/backend/content/108-more-about-databases/103-n-plus-one-problem.md

---------

Co-authored-by: Kamran Ahmed <kamranahmed.se@gmail.com>

src/data/roadmaps/backend/content/108-more-about-databases/103-n-plus-one-problem.md

@@ -4,4 +4,4 @@ The N+1 query problem happens when your code executes N additional query stateme
 
 Visit the following resources to learn more:
 
-- [In Detail Explanation of N+1 Problem](https://medium.com/doctolib/understanding-and-fixing-n-1-query-30623109fe89)
+- [What is the N+1 query problem and how to solve it?](https://planetscale.com/blog/what-is-n-1-query-problem-and-how-to-solve-it))