update the comments in SqlParser

SqlParser has been case-insensitive after https://github.com/apache/spark/commit/dab5439a083b5f771d5d5b462d0d517fa8e9aaf2 was merged

Author: CodingCat <zhunansjtu@gmail.com>

Closes #1275 from CodingCat/master and squashes the following commits:

17931cd [CodingCat] update the comments in SqlParser