Update Associativity Section in Expressions.Rmd

[micabo]

The ggplot2 example in the associativity section did not show that + is non-associative but rather that it is not commutative.

• Correct the ggplot2 example to shows the non-associativity of + (i.e. include a new example).
• Keep the original example as an additional point to show the non-commutative nature of +.

I assign the copyright of this contribution to Hadley Wickham