fix: resolved typo in storage setup documentation

Author: ThatDott

src/pages/[platform]/build-a-backend/storage/set-up-storage/index.mdx

@@ -787,7 +787,7 @@ class MyApp extends StatefulWidget {
 
 ### Upload your first file
 
-Next, let's a photo to the `picture-submissions/` path.
+Next, let's add a photo to the `picture-submissions/` path.
 
 <InlineFilter filters={["react", "react-native"]}>
 ```jsx