Fix Flutter `README.md` image sources (#146430)

fixes [Some images show "404 - page not found" in Flutter `README.md` ](https://github.com/flutter/flutter/issues/146419)

These images had their sources updated in https://github.com/flutter/website/pull/10336

### Before
![Screenshot 2024-04-08 at 18 19 30](https://github.com/flutter/flutter/assets/48603081/56549762-b7c8-41a1-89db-cd45cd98b35a)

### After
![Screenshot 2024-04-08 at 18 19 23](https://github.com/flutter/flutter/assets/48603081/35d5772f-1144-4166-99cb-fded79550762)
diff --git a/README.md b/README.md
index d07191c..ddce07b 100644
--- a/README.md
+++ b/README.md
@@ -62,7 +62,7 @@
 you're building for iOS ([Cupertino]) or other platforms ([Material]), along with
 support for customizing or creating entirely new visual components.
 
-<p align="center"><img src="https://github.com/flutter/website/blob/main/src/assets/images/docs/homepage/reflectly-hero-600px.png?raw=true" alt="Reflectly hero image"></p>
+<p align="center"><img src="https://github.com/flutter/website/blob/main/src/content/assets/images/docs/homepage/reflectly-hero-600px.png?raw=true" alt="Reflectly hero image"></p>
 
 ### Fast results
 
@@ -76,7 +76,7 @@
 JavaScript and WebAssembly for the web, as well as Intel x64 and ARM
 for desktop devices.
 
-<p align="center"><img src="https://github.com/flutter/website/blob/main/src/assets/images/docs/homepage/dart-diagram-small.png?raw=true" alt="Dart diagram"></p>
+<p align="center"><img src="https://github.com/flutter/website/blob/main/src/content/assets/images/docs/homepage/dart-diagram-small.png?raw=true" alt="Dart diagram"></p>
 
 ### Productive development