Remove trailing spaces (#755)

CONTRIBUTING.md

 # Contribution Guidelines
 Unfortunately, not every library, tool or framework can be considered for inclusion. The aim of Awesome PHP is to be a concise list of noteworthy and interesting software written in modern PHP. Therefore, suggested software should: 
 
-a) Be widely recommended regardless of personal opinion  
-b) Well known or discussed within the PHP community  
-c) Be unique in its approach or function  
+a) Be widely recommended regardless of personal opinion
+b) Well known or discussed within the PHP community
+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 opening a pull request, otherwise it may be closed without being reviewed.