[DOCS] Update StructType.scala

The example will throw error like
<console>:20: error: not found: value StructType

Need to add this line:
import org.apache.spark.sql.types._

Author: Kevin (Sangwoo) Kim <sangwookim.me@gmail.com>

Closes #10141 from swkimme/patch-1.

sql/catalyst/src/main/scala/org/apache/spark/sql/types/StructType.scala

@@ -40,6 +40,7 @@ import org.apache.spark.sql.catalyst.util.{DataTypeParser, LegacyTypeStringParse
  * Example:
  * {{{
  * import org.apache.spark.sql._
+ * import org.apache.spark.sql.types._
  *
  * val struct =
  *   StructType(