Docs: Fix typo in field name in aggregations

[populov]

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[elasticmachine]

Pinging @elastic/es-search-aggs

[nik9000]

Thanks for this @populov! I'll make sure everything is good and merge soon.

[nik9000]

It looks like the way this is written the aggregation returns empty with the wrong name. That seems like a strange thing, but it explain why the docs testing infrastructure didn't catch this. I'll merge and forward port to master and 6.x

[nik9000]

And 6.4.

[nik9000]

I've merged and forward ported to 6.4, 6.5, and master. Thanks again @populov!