Update README.md

[jonmcalder]

Grammar and spelling corrections

[codecov-io]

Codecov Report

Merging #18 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #18   +/-   ##
=======================================
  Coverage   66.66%   66.66%           
=======================================
  Files           2        2           
  Lines          18       18           
=======================================
  Hits           12       12           
  Misses          6        6

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[eddelbuettel]

Thank you!