fix: disjunctive_scheduling example had incorrect negation constraint

[tiptenbrink]

Hi, another tiny fix while getting everything running.

I'm pretty sure the negation of literal => s_y - s_x <= -p_y is as follows:

$$l => s_y - s_x <= -p_y$$ $$\neg l => s_y - s_x > -p_y$$ $$\neg l => s_y - s_x >= -p_y + 1$$ $$\neg l => -s_y + s_x <= p_y - 1$$

Also the literal was not actually negated in the second case, so I added that as well in the way I think you're supposed to do it.

[maartenflippo]

So what we are really after, is the following:

$$ \displaylines{ l_1 \rightarrow s_x + p_x \leq s_y \\ l_2 \rightarrow s_y + p_y \leq s_x } $$

Moving the variables to the left-hand side of the $\leq$, we have:

$$ \displaylines{ l_1 \rightarrow s_x - s_y \leq -p_x \\ l_2 \rightarrow s_y - s_x \leq -p_y } $$

Then we post the clause

$$ l_1 \lor l_2 $$

@tiptenbrink do you want to make these changes?

[tiptenbrink]

@maartenflippo this one was also ready