fix echidna ToC

Author: 0xalpharush

program-analysis/echidna/README.md

@@ -6,12 +6,12 @@ Watch our [Fuzzing workshop](https://www.youtube.com/watch?v=QofNQxW_K08&list=PL
 
 **Table of contents:**
 
-- [Introduction](./introduction/README.md): Introductory material to fuzzing and Echidna
-- [Basic](./basic/README.md): Learn the first steps on how to use Echidna
-- [Advanced](./advanced/README.md): Learn advanced features of Echidna
-- [Fuzzing tips](./fuzzing_tips.md): General fuzzing tips
-- [Frequently Asked Questions](./frequently_asked_questions.md): Answers to common questions about Echidna
-- [Exercises](./exercises/README.md): Exercises 
+- [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
+- [Exercises](exercises): Exercises 
 
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