Fix api docs loadconfigfile

[maxwell8888]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

The docs for rollup/dist/loadConfigFile incorrectly state that the options property of the object returned by loadConfigFile is an "inputOptions" object, when in fact it is an array of "inputOptions" objects. This PR fixes the text and code example in the docs.
Relevant stackoverflow question: https://stackoverflow.com/questions/62153013/rollup-js-use-rollup-config-js-in-js-api

[codecov]

Codecov Report

Merging #3757 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3757   +/-   ##
=======================================
  Coverage   97.02%   97.02%           
=======================================
  Files         185      185           
  Lines        6461     6461           
  Branches     1868     1868           
=======================================
  Hits         6269     6269           
  Misses        101      101           
  Partials       91       91           

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[lukastaegert]

Thanks for spotting and fixing this, always welcome!