fix naming error

Author: lohanidamodar

templates/flutter/lib/client.dart.twig

@@ -111,8 +111,8 @@ class Client {
             addHeader('user-agent', '${packageInfo.appName}/${packageInfo.version} $device');
         } else {
             // if web set withCredentials true to make cookies work
-            prefs = await SharedPreferences.getInstance();
-            addHeader('X-Fallback-Cookies', prefs.getString('cookieFallback') ?? '');
+            _prefs = await SharedPreferences.getInstance();
+            addHeader('X-Fallback-Cookies', _prefs.getString('cookieFallback') ?? '');
             this.http.options.extra['withCredentials'] = true;
         }
 
@@ -162,7 +162,7 @@ class Client {
                 if(cookieFallback != null) {
                     print('Appwrite is using localStorage for session management. Increase your security by adding a custom domain as your API endpoint.');
                     addHeader('X-Fallback-Cookies',cookieFallback);
-                    prefs.setString('cookieFallback', cookieFallback);
+                    _prefs.setString('cookieFallback', cookieFallback);
                 }
             }
             return res;