Replace usages of 'non-standard' quote marks in the Readme

[Moohan]

There are a couple of uses of the quote marks that word uses e.g. vignette(“opendatascot”) These won't work if copied into an R script. Simple to just replace them with 'standard' quote marks " instead.

P.s. if anyone knows the correct terminology for " vs “ and ” let me know!

[GordonBryden]

I've replaced the obvious ones on the readme - I'll work on routing out any others later.