[DOC][MINOR] Fix minor error in the code of graphx guide

## What changes were proposed in this pull request?

Fix minor error in the code "sketch of pregel implementation" of GraphX guide.
This fixed error relates to `[SPARK-12995][GraphX] Remove deprecate APIs from Pregel`

## How was this patch tested?

N/A

Closes apache#22780 from WeichenXu123/minor_doc_update1.

Authored-by: WeichenXu <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: WeichenXu123

docs/graphx-programming-guide.md

@@ -726,7 +726,7 @@ class GraphOps[VD, ED] {
     var g = mapVertices( (vid, vdata) => vprog(vid, vdata, initialMsg) ).cache()
 
     // compute the messages
-    var messages = g.mapReduceTriplets(sendMsg, mergeMsg)
+    var messages = GraphXUtils.mapReduceTriplets(g, sendMsg, mergeMsg)
     var activeMessages = messages.count()
     // Loop until no messages remain or maxIterations is achieved
     var i = 0