Fix missing variable declaration in data structures challenges (Russian) (#35640)

curriculum/challenges/russian/08-coding-interview-prep/data-structures/add-elements-at-a-specific-index-in-a-linked-list.russian.md

@@ -57,7 +57,7 @@ function LinkedList() {
     if(head === null){
         head = node;
     } else {
-        currentNode = head;
+        var currentNode = head;
 
         while(currentNode.next){
             currentNode  = currentNode.next;

curriculum/challenges/russian/08-coding-interview-prep/data-structures/remove-elements-from-a-linked-list-by-index.russian.md

@@ -60,7 +60,7 @@ function LinkedList() {
     if(head === null){
         head = node;
     } else {
-        currentNode = head;
+        var currentNode = head;
 
         while(currentNode.next){
             currentNode  = currentNode.next;

curriculum/challenges/russian/08-coding-interview-prep/data-structures/remove-elements-from-a-linked-list.russian.md

@@ -58,7 +58,7 @@ function LinkedList() {
     if(head === null){
         head = node;
     } else {
-        currentNode = head;
+        var currentNode = head;
 
         while(currentNode.next){
             currentNode  = currentNode.next;

curriculum/challenges/russian/08-coding-interview-prep/data-structures/search-within-a-linked-list.russian.md

@@ -60,7 +60,7 @@ function LinkedList() {
     if(head === null){
         head = node;
     } else {
-        currentNode = head;
+        var currentNode = head;
 
         while(currentNode.next){
             currentNode  = currentNode.next;