Corrected syntax for interface. (#32878)

Corrected syntax of interface. From Interface to interface (lowercase on first letter). 
If the first letter on the word interface is uppercase, this will produce an error on Visual Studio stating that: 

The type or namespace name 'Interface' could not be found (are you missing a using directive or an assembly reference?)

guide/english/csharp/interface/index.md

@@ -32,7 +32,7 @@ As interfaces do not implement any logic by themselves, they are a great tool fo
 
 Interface Example:
 ```csharp
-public Interface IUserFavoriteFood
+public interface IUserFavoriteFood
 {
   void AddFood();
   Task<User> EatFavoriteFood(int id);