README: reduce the size of the logo

README.md

@@ -1,5 +1,5 @@
 <p align="center">
-  <a href="https://arduinojson.org/"><img alt="ArduinoJson" src="https://arduinojson.org/images/logo.svg" /></a>
+  <a href="https://arduinojson.org/"><img alt="ArduinoJson" src="https://arduinojson.org/images/logo.svg" width="200" /></a>
 </p>
 
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