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/Platform.ts

@@ -44,7 +44,7 @@ export function getSafariVersion(): number {
   return parseInt(majorVersion[1], 10);
 }
 
-// Find the users platform. We use this to interpret the meta key
+// Find the user's platform. We use this to interpret the meta key
 // and ISO third level shifts.
 // http://stackoverflow.com/q/19877924/577598
 export const isMac = ['Macintosh', 'MacIntel', 'MacPPC', 'Mac68K'].includes(platform);