Fixed quadratic.cc documentation and code mismatch.

The hello world example, quadratic.cc and its description
in the tutorial were out of sync and led to some confusion
about the convergence behaviour. This change fixes the tex
file to match the code and discusses whats actually going
on when determining convergence.

Thanks to Nick Lewycky for reporting this.

Change-Id: Ic301d854c3f3d11e37a252e0832c00fb9e3a307c
1 file changed