chore(fix:style): fix Maven checkstyle

Each variable declaration must be in its own statement. [MultipleVariableDeclarations]

Author: sozelfist

src/main/java/com/thealgorithms/datastructures/trees/SplayTree.java

@@ -29,7 +29,8 @@ public class SplayTree {
 
     private static class Node {
         int key;
-        Node left, right;
+        Node left;
+        Node right;
 
         Node(int key) {
             this.key = key;