[SPARK-30192][SQL][FOLLOWUP] Rename SINGLETON to INSTANCE

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

This PR renames a variable `SINGLETON` to `INSTANCE`.

### Why are the changes needed?

This is a minor change for consistency with the other parts.

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

No.

### How was this patch tested?

Pass the existing tests.

Closes #27409 from dongjoon-hyun/SPARK-30192.

Authored-by: Dongjoon Hyun <dhyun@apple.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

sql/catalyst/src/main/java/org/apache/spark/sql/connector/catalog/TableChange.java

@@ -298,7 +298,7 @@ public int hashCode() {
   interface ColumnPosition {
 
     static ColumnPosition first() {
-      return First.SINGLETON;
+      return First.INSTANCE;
     }
 
     static ColumnPosition after(String column) {
@@ -312,7 +312,7 @@ static ColumnPosition after(String column) {
    * be the first one within the struct.
    */
   final class First implements ColumnPosition {
-    private static final First SINGLETON = new First();
+    private static final First INSTANCE = new First();
 
     private First() {}