[SQL][DOC] Wrong package name "scala.math.sql" in sql-programming-guide.md

In sql-programming-guide.md, there is a wrong package name "scala.math.sql".

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2873 from sarutak/wrong-packagename-fix and squashes the following commits:

4d5ecf4 [Kousuke Saruta] Fixed wrong package name in sql-programming-guide.md

docs/sql-programming-guide.md

@@ -1215,7 +1215,7 @@ import  org.apache.spark.sql._
 </tr>
 <tr>
   <td> <b>DecimalType</b> </td>
-  <td> scala.math.sql.BigDecimal </td>
+  <td> scala.math.BigDecimal </td>
   <td>
   DecimalType
   </td>