[REFACTOR] [Hacker Rank] Interview Preparation Kit: String Manipulation: Alternating Characters.

* Adjusted the interface to match what hackerrank expects.

Author: sir-gon

src/hackerrank/interview_preparation_kit/string_manipulation/alternating_characters.ts

@@ -2,7 +2,7 @@
  * @link Problem definition [[docs/hackerrank/interview_preparation_kit/string_manipulation/alternating-characters.md]]
  */
 
-export function alternatingCharacters(s: string): number {
+function alternatingCharacters(s: string): number {
   let last = '';
   let newString = '';