remove mention of ggplot in this lesson
missed an = used as assignment; change to <-
missed a mention of ggplot
key points had wrong info on ':' binary operator
modify key point on print()-ing
missed two cases of = used as assignment; change to <-
removed a duplicate line of text
change from using append() to c() to keep the lesson simple
remove 'paste()' calls for formatting output; give figures a prefix and name chunks; minor fixes as per github discussions