[css3-flexbox] a few straggling instances of "multiline" (no hyphen)

Nit: the flexbox ED has 2 instances of the word "multiline" (no hyphen),
at the end of Example 6 and in the caption for Figure 4.

It looks like every other instance of the term uses a hyphen
("multi-line"), so those two instances probably need a hyphen for
consistency's sake.

~Daniel