doc/doxygen: increase DOT_GRAPH_MAX_NODES to 275
doc/doxygen: increase DOT_GRAPH_MAX_NODES to 275

OlegHahm/RIOT,authmillenon/RIOT,kYc0o/RIOT,jasonatran/RIOT,miri64/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,miri64/RIOT,kYc0o/RIOT,ant9000/RIOT,kaspar030/RIOT,ant9000/RIOT,jasonatran/RIOT,kYc0o/RIOT,kaspar030/RIOT,authmillenon/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,jasonatran/RIOT,OlegHahm/RIOT,miri64/RIOT,kaspar030/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,ant9000/RIOT,kYc0o/RIOT,miri64/RIOT,jasonatran/RIOT,ant9000/RIOT,RIOT-OS/RIOT,miri64/RIOT,ant9000/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,authmillenon/RIOT,OlegHahm/RIOT,authmillenon/RIOT,authmillenon/RIOT