docs: 📚️added missing selector function in async components

[lourensdev]

Following the documentation with a fresh install, the example with a custom loader does not work. A build error occurs indicating the selector function is missing. This fixes it.

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[lourensdev]

@all-contributors please add @lourensdev for documentation

[coveralls]

Pull Request Test Coverage Report for Build 1022451278

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.678%

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Totals
Change from base Build 879499260:	0.0%
Covered Lines:	249
Relevant Lines:	249

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💛 - Coveralls