Fix Flutter README.md image sources (#146430)

fixes [Some images show "404 - page not found" in Flutter `README.md` ](flutter/flutter#146419)

These images had their sources updated in flutter/website#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)

Author: TahaTesser

README.md

@@ -62,7 +62,7 @@ graphics, video, text, and controls without limitation. Flutter includes a full
 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 @@ compilation to 32-bit and 64-bit ARM machine code for iOS and Android,
 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