fix: add jestOpenAPI function export to type declaration file

[BenGu3]

The default function was missing in the Typescript declarations for jest-openapi (similar to #61 which is fixed by these two lines).
This fixes #73.

[codecov]

Codecov Report

Merging #74 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #74   +/-   ##
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  Coverage   95.41%   95.41%           
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  Files          33       33           
  Lines         633      633           
  Branches       51       51           
=======================================
  Hits          604      604           
  Misses         27       27           
  Partials        2        2           

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[rwalle61]

Thanks very much!

(I think I spotted this before I published the module, but accidentally didn't commit it in the midst of all the changes. Thanks for catching and fixing it)