fix links

Author: 0xalpharush

program-analysis/echidna/README.md

@@ -9,8 +9,8 @@ Watch our [Fuzzing workshop](https://www.youtube.com/watch?v=QofNQxW_K08&list=PL
 - [Introduction](introduction): Introductory material to fuzzing and Echidna
 - [Basic](basic): Learn the first steps on how to use Echidna
 - [Advanced](advanced): Learn advanced features of Echidna
-- [Fuzzing tips](fuzzing_tips): General fuzzing tips
-- [Frequently Asked Questions](frequently_asked_questions): Answers to common questions about Echidna
+- [Fuzzing tips](./fuzzing_tips.md): General fuzzing tips
+- [Frequently Asked Questions](./frequently_asked_questions.md): Answers to common questions about Echidna
 - [Exercises](exercises): Exercises 
 
 Join the team on Slack at: https://empireslacking.herokuapp.com/ #ethereum