[SAGA-RG] just a couple of glitches in the Saga specs

Hi Andre, just a couple of minor glitches this time, and nothing debatable, I think. The context class description still refers to the set_defaults method, which no longer exists: "(see documentation to the set_defaults() method." (at the bottom of page 81). Notes at remove_context talk about "parameter values", that should be "attribute values". Cheers, Ceriel