proofreading fixes to translate toolkit style guide

typos, mostly, hardly seemed worth creating a separate branch for this... but probably would have been better, as this pull request apparently includes some old changes from last year that Julen did pick up, but which must have come from a different repo of mine (and thus have different hashes, and so are not excluded).  I should probably have been tipped off by the spurious conflicts I had when I pulled from translate:master - they are resolved in the merge commit in favor of the latest translate:master versions.  I can clean up my repo (re-forking seems best) and re-submit tomorrow, but leaving it as is for now in case you just want to cherry-pick the changes to styleguide.rst ...