Section 1.4 end, distributive rule math typo

[vitanza]

At the very end of section 1.4, in the box "More Number Properties with Vectors", there is a math typo in the first distributive rule formula. The first multiplication should be an addition.

Currently:

(n * m) * v  =  n * v  +  m * v

Corrected:

(n + m) * v  =  n * v  +  m * v

[Daniel Shiffman]

duplicate of #298