Check for array length in tail function to return the correct value for single-value arrays.

snippets/tail-of-list.md

@@ -3,6 +3,7 @@
 Return `arr.slice(1)`.
 
 ```js
-const tail = arr => arr.slice(1);
+const tail = arr => arr.length > 1 ? arr.slice(1) : arr;
 // tail([1,2,3]) -> [2,3]
+// tail([1]) -> [1]
 ```