Fix BackgroundFill class extensions

Author: mircoianese

library/src/main/java/com/pengrad/telegrambot/model/chatbackground/BackgroundFillFreeformGradient.java

@@ -2,7 +2,7 @@
 
 import java.util.Arrays;
 
-public class BackgroundFillFreeformGradient extends BackgroundType {
+public class BackgroundFillFreeformGradient extends BackgroundFill {
 
     public static final String TYPE = "freeform_gradient";
 

library/src/main/java/com/pengrad/telegrambot/model/chatbackground/BackgroundFillGradient.java

@@ -2,7 +2,7 @@
 
 import java.util.Objects;
 
-public class BackgroundFillGradient extends BackgroundType {
+public class BackgroundFillGradient extends BackgroundFill {
 
     public static final String TYPE = "gradient";
 

library/src/main/java/com/pengrad/telegrambot/model/chatbackground/BackgroundFillSolid.java

@@ -2,7 +2,7 @@
 
 import java.util.Objects;
 
-public class BackgroundFillSolid extends BackgroundType {
+public class BackgroundFillSolid extends BackgroundFill {
 
     public static final String TYPE = "solid";