[SPARK-36273][SHUFFLE] Fix identical values comparison

This commit fixes the use of the "o.appAttemptId" variable instead of the mistaken "appAttemptId" variable. The current situation is a comparison of identical values. Jira issue report SPARK-36273.

### What changes were proposed in this pull request?
This is a patch for SPARK-35546 which is needed for push-based shuffle.

### Why are the changes needed?
A very minor fix of adding the reference from the other "FinalizeShuffleMerge".

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
No unit tests were added. It's a pretty logical change.

Closes #33493 from almogtavor/patch-1.

Authored-by: Almog Tavor <70065337+almogtavor@users.noreply.github.com>
Signed-off-by: Sean Owen <srowen@gmail.com>
(cherry picked from commit 530c8ad)
Signed-off-by: Sean Owen <srowen@gmail.com>

common/network-shuffle/src/main/java/org/apache/spark/network/shuffle/protocol/FinalizeShuffleMerge.java

@@ -68,7 +68,7 @@ public boolean equals(Object other) {
     if (other != null && other instanceof FinalizeShuffleMerge) {
       FinalizeShuffleMerge o = (FinalizeShuffleMerge) other;
       return Objects.equal(appId, o.appId)
-        && appAttemptId == appAttemptId
+        && appAttemptId == o.appAttemptId
         && shuffleId == o.shuffleId;
     }
     return false;