IssueLabelApp: Rewrite javadoc in English

이것도 위와 같은 의도였구요, 메소드의 기능을 확실히 명시한다는 의미에서 

When a new label is added, this method encodes the label's fields: {@link IssueLabel#id}, {@link IssueLabel#name}, {@link IssueLabel#color}, and {@link IssueLabel#category} into {@code application/json}, and includes them in the body of the 201 Created response. But if the client cannot accept {@code application/json}, it returns the 201 Created with no response body.

라고 하면 될 것 같아요.