Added the sample review changes

Author: Yuvaraj-Gajaraj

lib/samples/chart/cartesian_charts/axis_types/date_time_category/default_date_time_category.dart

@@ -61,25 +61,25 @@ class _DateTimeCategoryDefaultState extends SampleViewState {
             ),
             plotBands: <PlotBand>[
               PlotBand(
-                  start: DateTime(2017, 12, 22),
-                  end: DateTime(2017, 12, 27),
-                  textAngle: 0,
-                  verticalTextAlignment: TextAnchor.start,
-                  verticalTextPadding: '%-5',
-                  text: 'Christmas Offer \nDec 2017',
-                  textStyle: TextStyle(color: labelColor, fontSize: 13),
-                  color: const Color.fromRGBO(50, 198, 255, 1),
-                  opacity: 0.3),
+                start: DateTime(2017, 12, 22),
+                end: DateTime(2017, 12, 27),
+                textAngle: 0,
+                verticalTextAlignment: TextAnchor.start,
+                verticalTextPadding: '%-5',
+                text: 'Christmas Offer \nDec 2017',
+                textStyle: TextStyle(color: labelColor, fontSize: 13),
+                color: const Color.fromRGBO(50, 198, 255, 1).withOpacity(0.3),
+              ),
               PlotBand(
-                  textAngle: 0,
-                  start: DateTime(2018, 1, 2),
-                  end: DateTime(2018, 1, 4),
-                  verticalTextAlignment: TextAnchor.start,
-                  verticalTextPadding: '%-5',
-                  text: 'New Year Offer \nJan 2018',
-                  textStyle: TextStyle(color: labelColor, fontSize: 13),
-                  color: Colors.pink,
-                  opacity: 0.2),
+                textAngle: 0,
+                start: DateTime(2018, 1, 2),
+                end: DateTime(2018, 1, 4),
+                verticalTextAlignment: TextAnchor.start,
+                verticalTextPadding: '%-5',
+                text: 'New Year Offer \nJan 2018',
+                textStyle: TextStyle(color: labelColor, fontSize: 13),
+                color: Colors.pink.withOpacity(0.2),
+              ),
             ]),
         tooltipBehavior: _tooltipBehavior,
         title: