Update index.d.ts

[arlair]

Isn't this third parameter, notSetValue, meant to be optional?
https://github.com/prescottprue/react-redux-firebase/blob/master/docs/api/helpers.md#getval

Description

Check List

If not relevant to pull request, check off as complete

• [ x] All tests passing
• [ x] Docs updated with any changes or examples if applicable
• [ x] Added tests to ensure new feature(s) work properly

Relevant Issues

[codecov]

Codecov Report

Merging #521 into master will not change coverage.
The diff coverage is n/a.

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##           master     #521   +/-   ##
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  Lines        2519     2519           
  Branches      399      399           
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  Hits         2309     2309           
  Misses        210      210