Add missing, required, xsi:type definition to argument node

[robaimes]

Adds the missing xsi:type argument for the <argument> node. Without the xsi:type present, errors are shown during DOM validation:

Example error

In Filesystem.php line 168:
                                                                                                              
  Invalid Document                                                                                            
  Element 'argument': The type definition is abstract.                                                        
  Line: 11                                                                                                    
                                                                                                              
  Internal error: xmlSchemaXPathProcessHistory, The state object to be removed is not the first in the list.  
  Line: 10                                                                                                    
                                                                                                              
  Internal error: xmlSchemaValidateChildElem, calling xmlRegExecPushString2().                                
  Line: 104                                                                                                   
                                                                                                              
  Internal error: xmlSchemaValidateElem, calling xmlSchemaStreamValidateChildElement().                       
  Line: 104                                                                                                   
                                                                                                              
  Internal error: xmlSchemaDocWalk, calling xmlSchemaValidateElem().                                          
  Line: 104                                                                                                                            

[magefan]

@robaimes , thank you for your contribution