docs(common): fix grammar in Unicode and platform comments

Use "a Unicode" instead of "an Unicode" on IUnicodeService.register,
and "user's platform" instead of "users platform" above the platform
detection exports.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/common/services/Services.ts

@@ -344,7 +344,7 @@ export type UnicodeCharWidth = 0 | 1 | 2;
 export const IUnicodeService = createDecorator<IUnicodeService>('UnicodeService');
 export interface IUnicodeService {
   serviceBrand: undefined;
-  /** Register an Unicode version provider. */
+  /** Register a Unicode version provider. */
   register(provider: IUnicodeVersionProvider): void;
   /** Registered Unicode versions. */
   readonly versions: string[];