Fix a few typos in the shiny tutorial
Slight changes to two links to R-manual, for consistency

 - Most links were to .../R-manual/R-devel/... but two were to
   .../R-manual/R-patched/...; changed the R-patched ones to R-devel,
   for consistency.

 - Changed "this article on [R startup]" to have the "this article on"
   included in the underlined bit; seemed a bit more clear.