docs: fix exercise 5 and 6 names

Exercise names were swapped.

Author: ChmielewskiKamil

program-analysis/echidna/README.md

@@ -31,8 +31,8 @@ Watch our [Fuzzing workshop](https://www.youtube.com/watch?v=QofNQxW_K08&list=PL
   - [Exercise 2](./Exercise-2.md): Testing access control
   - [Exercise 3](./Exercise-3.md): Testing with custom initialization
   - [Exercise 4](./Exercise-4.md): Testing with `assert`
-  - [Exercise 5](./Exercise-5.md): Solving Damn Vulnerable DeFi - Unstoppable
-  - [Exercise 6](./Exercise-6.md): Solving Damn Vulnerable DeFi - Naive Receiver
+  - [Exercise 5](./Exercise-5.md): Solving Damn Vulnerable DeFi - Naive Receiver
+  - [Exercise 6](./Exercise-6.md): Solving Damn Vulnerable DeFi - Unstoppable
   - [Exercise 7](./Exercise-7.md): Solving Damn Vulnerable DeFi - Side Entrance
   - [Exercise 8](./Exercise-8.md): Solving Damn Vulnerable DeFi - The Rewarder