tutorial ffi: add emphasis

Add emphasis to 'target' in the examples so the string 'target' doesn't
get confused with concrete syntax:

    %include target "x"
tutorial: fix typos, add additional step to proof

An additional step is needed to prove plusredZ_S, namely compute.
tutorial: update interactive proof

Update interactive proof so it matches what one would get at the moment.
Merge pull request #519 from reynir/master

Update tutorial