Update README.md

Typo fix

Author: ajmeese7

README.md

@@ -272,7 +272,7 @@ appId| int | The ID of the game you want the user's achievements in | Yes |
 > Example Output: [GetGlobalAchievementPercentagesForApp](./examples/user/stats/GetGlobalAchievementPercentageForApp.txt)
 
 #### GetUserStatsForGame
-Returns a list of achievements for this user by app IX.
+Returns a list of achievements for this user by app ID.
 
 ##### Arguments