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

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/SqlParser.scala

@@ -35,7 +35,6 @@ import org.apache.spark.sql.catalyst.types._
  *
  * Limitations:
  *  - Only supports a very limited subset of SQL.
- *  - Keywords must be capital.
  *
  * This is currently included mostly for illustrative purposes.  Users wanting more complete support
  * for a SQL like language should checkout the HiveQL support in the sql/hive sub-project.