corrected equivalent long-form snippet in readme.md

[gdmcbain]

• data is a list so need an extra nesting of iteration
• no underscore in isinstance
• variable-name is letter not maybe_letter (or standardize the other way)

Checked in Python 3.6.

[codecov[bot]]

Codecov Report

Merging #13 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #13   +/-   ##
======================================
  Coverage    98.6%   98.6%           
======================================
  Files          18      18           
  Lines        1006    1006           
  Branches       95      95           
======================================
  Hits          992     992           
  Misses         14      14

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