Material conditional is also called material implication

[david-a-wheeler]

Clarify early on that this definition of the conditional is the material conditional (readers might otherwise not realize this), and make it clear that an alternative term is "material implication". The term material implication is also widely used, and it's important that students see common terms so that they can begin to find the relevant literature.

Signed-off-by: David A. Wheeler [email protected]