fix 965 filename

Author: rustykitty

965-univalued-binary-tree.cpp renamed to 0965-univalued-binary-tree.cpp

@@ -28,4 +28,4 @@ class Solution {
         if (root->val != val) return false;
         return isUnivalTree(root->left) && isUnivalTree(root->right);
     }
-};
+};