Update CONTRIBUTING.md
Update CONTRIBUTING.md

Fixed some grammar issues.
ROBOTIS-GIT/DynamixelSDK,ROBOTIS-GIT/DynamixelSDK,ROBOTIS-GIT/DynamixelSDK,ROBOTIS-GIT/DynamixelSDK,ROBOTIS-GIT/DynamixelSDK,ROBOTIS-GIT/DynamixelSDK,ROBOTIS-GIT/DynamixelSDK