DOC fix image path in swc-shell

bullet 1 in issue #67

	modified:   _includes/guide-shell/create/lesson.html
	modified:   _includes/guide-shell/filedir/challenges.html
	modified:   _includes/guide-shell/filedir/lesson.html
	modified:   _includes/guide-shell/find/lesson.html
	modified:   _includes/guide-shell/intro/lesson.html
	modified:   _includes/guide-shell/pipefilter/lesson.html