Update library_add_new_module.md (#1702)

doc/library_add_new_module.md

 OpenPose Library - Steps to Add a New Module
 ====================================
 
-## Developping Steps
+## Developing Steps
 In order to add a new module, these are the recommended steps in order to develop it:
 
 1. Create a folder with its name in the `experimental/` module, e.g., `experimental/hair/`.