Update README.md

Author: martinkeen

README.md

@@ -8,7 +8,7 @@ This project is a set of training materials and labs for a "Getting Started" lev
 
 There are two sets of materials available
 
-- [Chapters](https://github.com/openmainframeproject/cobol-programming-course/blob/master/COBOL%20Programming%20with%20VSCode.pdf) which contain the lessons, broken up into chapters.
+- [Chapters](https://github.com/openmainframeproject/cobol-programming-course/blob/master/COBOL%20Programming%20with%20VSCode.md) which contain the lessons, broken up into chapters.
 - [Labs](https://github.com/openmainframeproject/cobol-programming-course/tree/master/Labs) which contain the source code used in the chapters.
 
 ## Build