It seems the page you referring to has a typo. The node is actually called cameracalibrator.py. Also you added spaces in the ROS parameter assignment and the topic namespace, so pay attention.
rosrun camera_calibration cameracalibrator.py --size 8x6 --square 0.025 image:=/image_raw

The easiest way to avoid such problems is to type in everything yourself (do not copy/paste) and use bash-completion by pressing Tab. ROS command-line tools have pretty good completion support for most of parameters. In case of rosrun the completion is very helpful for package names (first parameter) and node names (second parameter); note, however, to make this to work for your own nodes the script should be actually marked as executable on the file system with, e.g. chmod a+x <script name>