Fixed Decoration error

Line 98's decoration had a call () on it preventing the code from working, I removed the parenthesis and now it works.

Author: Jorden-Quast

lib/src/form_builder_image_picker.dart

@@ -95,7 +95,7 @@ class FormBuilderImagePicker extends FormBuilderField<List<dynamic>> {
             final primaryColor = theme.primaryColor;
 
             return InputDecorator(
-              decoration: state.decoration(),
+              decoration: state.decoration,
               child: Container(
                 height: previewHeight,
                 child: ListView(