Fixed incorrect XML docs

Author: Sergio0694

Microsoft.Toolkit.HighPerformance/Extensions/StringExtensions.cs

@@ -109,7 +109,7 @@ public static int Count(this string text, char c)
         /// {
         ///     // Access the index and value of each item here...
         ///     int index = item.Index;
-        ///     string value = item.Value;
+        ///     char value = item.Value;
         /// }
         /// </code>
         /// The compiler will take care of properly setting up the <see langword="foreach"/> loop with the type returned from this method.