Update nested_queries.md

[jcayetano]

Fixing typo on moving_rollup function

What does this PR do? What is the motivation?

Merge instructions

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• Ready for merge

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Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/cayetano.delgado/fixing_typo/metrics/nested_queries

[michaelcretzman]

@jcayetano
Approved.
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