Corrects a typo in specifying arguments

Author: denx-b

README.md

@@ -143,7 +143,7 @@ $image->save('image2.jpg');
 
 ```php
 $image = new ImageResize('image.jpg');
-$image->crop(200, 200, ImageResize::CROPCENTER);
+$image->crop(200, 200, true, ImageResize::CROPCENTER);
 $image->save('image2.jpg');
 ```
 
@@ -154,15 +154,15 @@ For instance passing the crop mode `CROPTOP` will result as 100px taken off the
 
 ```php
 $image = new ImageResize('image.jpg');
-$image->crop(200, 200, ImageResize::CROPTOP);
+$image->crop(200, 200, true, ImageResize::CROPTOP);
 $image->save('image2.jpg');
 ```
 
 On the contrary passing the crop mode `CROPBOTTOM` will result as 100px taken off the top leaving you with 200px × 200px.
 
 ```php
 $image = new ImageResize('image.jpg');
-$image->crop(200, 200, ImageResize::CROPBOTTOM);
+$image->crop(200, 200, true, ImageResize::CROPBOTTOM);
 $image->save('image2.jpg');
 ```