updated cfg file name to prevent rebuilding every time

Hi there 

I changed the name of the cfg file to prevent the regeneration of the header files and the resulting recompiling of the ssf package.

exit(gen.generate(PACKAGE, "dynamic_tutorials", "Tutorials"))

The third parameter is a name prefix the generated files will get (e.g. "<name>Config.h"
The third parameter should be equal to the cfg file name, without extension. Otherwise the libraries will be generated in every build, forcing a recompilation of the nodes which use them. 

(http://wiki.ros.org/dynamic_reconfigure/Tutorials/HowToWriteYourFirstCfgFile)