Update src/services/mapCode.ts

Co-authored-by: Sheetal Nandi <shkamat@microsoft.com>

Author: zkat

src/services/mapCode.ts

@@ -50,7 +50,7 @@ export function mapCode(
     host: LanguageServiceHost,
     formatContext: formatting.FormatContext,
     preferences: UserPreferences,
-): FileTextChanges[] {
+): readonly FileTextChanges[] {
     try {
         return textChanges.ChangeTracker.with(
             { host, formatContext, preferences },