Fixed typo in doc

doc/library_introduction.md

@@ -3,7 +3,7 @@ OpenPose C++ API - Introduction
 
 The C++ API is ideal if you want to e.g. change internal functions and/or extend the OpenPose functionality. In order to learn the basics:
 
-1. Take a look at the [library Quick Start section](../README#quick-start) from the README.
+1. Take a look at the [library Quick Start section](../README.md#quick-start) from the README.
 2. OpenPose Overview: Learn the basics about the library source code in [doc/library_overview.md](./library_overview.md).
 3. Extending Functionality: Learn how to extend the library in [doc/library_extend_functionality.md](./library_extend_functionality.md).
 4. Adding An Extra Module: Learn how to add an extra module in [doc/library_add_new_module.md](./library_add_new_module.md).