Updated readme

README.md

-# <img src="http://www.cesanta.com/hubfs/www.cesanta.com/Images/mongoose_logo.png" width="64" height="64"> Mongoose Embedded Web Server
+# <img src="http://www.cesanta.com/hubfs/www.cesanta.com/Images/mongoose_library.png" width="64" height="64"> Mongoose Embedded Web Server
 
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