fix: fixed the failing test pipeline errors

Author: KaranChadha10

exercises/practice/parallel-letter-frequency/.meta/Example.cs

@@ -1,6 +1,6 @@
-public class ParallelLetterFrequency
+public static class ParallelLetterFrequency
 {
-    public async Task<Dictionary<char, int>> Calculate(IEnumerable<string> texts)
+    public static async Task<Dictionary<char, int>> Calculate(IEnumerable<string> texts)  // Add static here
     {
         var textList = texts.ToList();
 
@@ -17,15 +17,15 @@ public async Task<Dictionary<char, int>> Calculate(IEnumerable<string> texts)
         return MergeDictionaries(results);
     }
 
-    private Dictionary<char, int> CountLettersInText(string text)
+    private static Dictionary<char, int> CountLettersInText(string text)  // Add static here
     {
         return text.ToLower()
                    .Where(char.IsLetter)
                    .GroupBy(c => c)
                    .ToDictionary(g => g.Key, g => g.Count());
     }
 
-    private Dictionary<char, int> MergeDictionaries(IEnumerable<Dictionary<char, int>> dictionaries)
+    private static Dictionary<char, int> MergeDictionaries(IEnumerable<Dictionary<char, int>> dictionaries)  // Add static here
     {
         var result = new Dictionary<char, int>();