Chapter 8, rename case sensitive filename

This should fix the following problem (loving CI)...
ci-pharo-contribution@inria.fr wrote:
> See <https://ci.inria.fr/pharo-contribution/job/PharoLaserGame/28/changes>
> Changes:
> [btc] Completed review of Chapter 8 Improving Our Model
> ------------------------------------------
> LaTeX Warning: File `figures/7-isSegmentOnThree.pdf' not found on input line 81

The problem is inconsistent upper/lowercase filename  "7-i" versus "7-I".  
    7-isSegmentOnOne.pdf   
    7-IsSegmentOnThree.pdf   
    7-isSegmentOnTwo.pdf

Actually it compiles okay on Mavericks, so its apparently Mavericks is case-insensitive while the CI server is case-sensitive.  Something to watch out for.