Fix typo

CONTRIBUTING.md

 c) Be unique in its approach or function  
 d) Fill a niche gap in the market  
 
-Self-promotion is frowned upon, so please consider seriously whether your project meets the criteria before open a pull request, otherwise it may be closed without being reviewed.
+Self-promotion is frowned upon, so please consider seriously whether your project meets the criteria before opening a pull request, otherwise it may be closed without being reviewed.
 
 Also, please ensure your pull request adheres to the following guidelines: