docs: center & resize demo screenshot to 300 px

Author: EvMossan

README.md

@@ -5,7 +5,10 @@
 > "Where should this go in Jyväskylä?"
 > Ask in English or Finnish, or snap a photo—the bot knows the answer.
 
-![Chatbot demo](docs/demo_thumb.png)
+<p align="center">
+  <img src="docs/demo.png" alt="Demo screenshot" width="300">
+</p>
+
 
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