[MINOR][SS] Fix setTimeoutTimestamp doc

### What changes were proposed in this pull request?

This patch updates the API doc of `setTimeoutTimestamp` of `GroupState`.

### Why are the changes needed?

Update incorrect API doc.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

Doc change only.

Closes #39958 from viirya/fix_group_state.

Authored-by: Liang-Chi Hsieh <viirya@gmail.com>
Signed-off-by: Liang-Chi Hsieh <viirya@gmail.com>

Author: viirya

sql/core/src/main/scala/org/apache/spark/sql/streaming/GroupState.scala

@@ -259,7 +259,7 @@ trait GroupState[S] extends LogicalGroupState[S] {
   @throws[IllegalArgumentException](
     "if 'timestampMs' is not positive or less than the current watermark in a streaming query")
   @throws[UnsupportedOperationException](
-    "if processing time timeout has not been enabled in [map|flatMap]GroupsWithState")
+    "if event time timeout has not been enabled in [map|flatMap]GroupsWithState")
   def setTimeoutTimestamp(timestampMs: Long): Unit