Edit steps to Epsilon-Delta proof

[APEXCalculus]

In the steps leading to Equation (1.2.3) in
Example 1.2.6, we establish that x^2+x-1 could be negative, yet also state that
x^2+x-1 < 5 implies
1/(x^2+x-1) > 1/5.

We need to add abs. values to some of these steps.

[sean-fitzpatrick]

Agreed! We usually don't teach this section, so I haven't looked carefully at the proofs.

We have the \abs{} macro for adding absolute values.