Update image understanding model (#4122)

Update model used in image generation code sample

Author: anirudh161

genai/text-generation/textgen-with-txt-img.js

@@ -38,7 +38,7 @@ async function generateContent(
   };
 
   const response = await ai.models.generateContent({
-    model: 'gemini-2.0-flash',
+    model: 'gemini-2.5-flash',
     contents: [image, 'What is shown in this image?'],
   });