fixed typo in README.md

[devmanbud]

isBoolean(str [, options]) | check if the string is a boolean.
options is an object which defaults to { loose: false }. If loose is set to false, the validator will strictly match ['true', 'false', '0', '1']. If loose is set to true, the validator will also match 'yes', 'no', and will match a valid boolean string of any case. (e.g.: ['true', 'True', 'TRUE']).

had a typo, fixed in README.md file.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.95%. Comparing base (b958bd7) to head (41b465c).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2371      +/-   ##
==========================================
- Coverage   99.95%   99.95%   -0.01%     
==========================================
  Files         107      107              
  Lines        2454     2449       -5     
  Branches      619      619              
==========================================
- Hits         2453     2448       -5     
  Partials        1        1              

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