added slack group link.

README.md

@@ -5,8 +5,7 @@
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 ## Notes
 
@@ -37,5 +36,5 @@
   - [Chapter 16. Templates and Generic Programming](ch16/README.md)
 - Part IV:  Advanced Topics
   - [Chapter 17. Specialized Library Facilities](ch17/README.md)
-  - [Chapter 18. Tools for Large Programs](ch18)
+  - [Chapter 18. Tools for Large Programs](ch18/README.md)
   - Chapter 19. Specialized Tools and Techniques