Modified meta_curriculum JSON reading loop to only deal with .json (#2651)

* Modified meta_curriculum JSON reading loop

* Switched quote styling

* Removed whitespace

ml-agents/mlagents/trainers/meta_curriculum.py

 
         try:
             for curriculum_filename in os.listdir(curriculum_folder):
+                # This process requires JSON files
+                if not curriculum_filename.lower().endswith(".json"):
+                    continue
                 brain_name = curriculum_filename.split(".")[0]
                 curriculum_filepath = os.path.join(
                     curriculum_folder, curriculum_filename