Minor edits to README (e.g. punctuation, spelling)

[batpigandme]

Mostly minor phrasing, other edits also noted in package review.

• Removed sentence re. reordering being better-illustrated, as airquality is now used in the first example
• vis_dat(airquality) is numeric and integer values, when I run it. Thus, I've swapped numeric in for character as it was described.
  • Added canonical link to the mi package, since you do so for wakefield

[codecov-io]

Codecov Report

Merging #38 into master will not impact coverage.

@@           Coverage Diff           @@
##           master      #38   +/-   ##
=======================================
  Coverage   99.24%   99.24%           
=======================================
  Files           9        9           
  Lines         265      265           
=======================================
  Hits          263      263           
  Misses          2        2

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