fix: fix setFieldData function calling in example from README.md

[JonatanSalas]

This is only a fix related to how setFieldData function is intended to work.

When using this library in combination with React Final Form, I used the setFieldData call as in the example presented in the README.md, but it wasn't working as expected, after reading the code of this mutator, I found that it checks whether the field exists, so, if call this before a field is already registered, the mutator call won't work.

Hope this helps for people trying to use this library.

[codecov]

Codecov Report

Merging #5 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master     #5   +/-   ##
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  Coverage     100%   100%           
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  Files           1      1           
  Lines           5      5           
  Branches        1      1           
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  Hits            5      5

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