Change from uppercase to lowercase the first letter of the progress prop...

The progress property definition is the only one beginning with an uppercase letter, so I changed it for a lowercase letter:

	TASK PROPERTIES
	---------------
	status                          contents of Status task column  (read-only)
	icon                            eg 'gtk-open'
	count                           text to show in Count task column
	folder                          text to show in Folder task column
	item                            text to show in Item task column
	to                              text to show in To task column
	progress                        Progress percent (1..100) or '' to pulse
	total                           text to show in Total task column
	curspeed                        text to show in Current task column
	curremain                       text to show in CRemain task column
	avgspeed                        text to show in Average task column
	avgremain                       text to show in Remain task column
	elapsed                         contents of Elapsed task column (read-only)
	started                         contents of Started task column (read-only)
	queue_state                     run|pause|queue|stop
	popup_handler                   COMMAND  command to show a custom task dialog