Fixes bad image path

Author: epsilonrt

Doxyfile

@@ -61,7 +61,7 @@ PROJECT_BRIEF          = "Modbus library for Arduino"
 # pixels and the maximum width should not exceed 200 pixels. Doxygen will copy
 # the logo to the output directory.
 
-PROJECT_LOGO           = extra/images/modbus.png
+PROJECT_LOGO           = https://github.com/epsilonrt/modbus-arduino/raw/master/extras/modbus.png
 
 # The OUTPUT_DIRECTORY tag is used to specify the (relative or absolute) path
 # into which the generated documentation will be written. If a relative path is