Update flutter.json

Mainly sizedbox is used for giving flexible spaces. Stack has a different purpose.

Author: tusharhow

data/flutter.json

@@ -148,7 +148,7 @@
                 },
                 {
                     "definition": "স্পেস নিতে",
-                    "code": "Stack"
+                    "code": "SizedBox"
                 },
                 {
                     "definition": "জেশ্চার অপশন",