Update index.d.ts

[matiosfree]

#864 FormProps interface doesn't support custom props.

[codesandbox-ci]

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Latest deployment of this branch, based on commit ac0198c:

Sandbox	Source
React Final Form - Simple Example	Configuration
React Final Form - Synchronous Record Level Validation	Configuration
React Final Form - Synchronous Field Level Validation	Configuration
React Final Form - Submission Errors	Configuration
React Final Form - Subscriptions Example	Configuration

[codecov]

Codecov Report

Merging #867 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #867   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           17        17           
  Lines          253       253           
  Branches        59        59           
=========================================
  Hits           253       253           

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[erikras]

Published in v6.5.2.