docs/TestingMinimalProgram: Fix typo in config file example

Signed-off-by: Diego Biurrun <diego@biurrun.de>
docs/BuildingS2E: Remove instructions for manually building S2E.

The instructions are outdated wrt the commands actually used in
the Makefile, so refer to the Makefile instead.

Signed-off-by: Diego Biurrun <diego@biurrun.de>