[SQL][MINOR] XPathDouble prettyPrint should say 'double' not 'float'

## What changes were proposed in this pull request?

It looks like this was incorrectly copied from `XPathFloat` in the class above.

## How was this patch tested?

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)
(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Eric Liang <ekhliang@gmail.com>

Closes #20730 from ericl/fix-typo-xpath.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/xml/xpath.scala

@@ -160,7 +160,7 @@ case class XPathFloat(xml: Expression, path: Expression) extends XPathExtract {
   """)
 // scalastyle:on line.size.limit
 case class XPathDouble(xml: Expression, path: Expression) extends XPathExtract {
-  override def prettyName: String = "xpath_float"
+  override def prettyName: String = "xpath_double"
   override def dataType: DataType = DoubleType
 
   override def nullSafeEval(xml: Any, path: Any): Any = {