If you mean the description shown on the screen its in the message bundle file (.properties). YOu can add your own and override that particular message.
Yes i know that the propertie file is the target but i would like to determine the file exactly which contain this information(the word "Start Workflow")
here yu can find that C:\Alfresco\tomcat\webapps\share\WEB-INF\classes\alfresco\site-webscripts\org\alfresco\components\workflow\start-workflow.get.properties