update a few things in example script

[iantaylor-NOAA]

I just worked through /examples/VAST--single-species_example.Rmd and found it very informative and clearly put together. Here are three minor issues that occurred to me along the way:

• The PDF is appealing compared to the Rmd file because it includes inline images and expected output. However, the PDF doesn't including the crucial function calls in the section "Data-frame for catch-rate data".
• Consider adding "pander" to the packages installed in the "Getting started" section or make it a dependency of VAST. I presume pander is just used for formatting the PDF, but for the naive user copying and pasting commands, it would be convenient to avoid the error message that the package isn't available.
• Increment the date to reflect most recent edits to the file.