Fix aws.billing.EstimatedCharges field name

[kaiyan-sheng]

What does this PR do?

This PR is to fix the aws.billing.EstimatedCharges field name.

closes: #1592

[elasticmachine]

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• Start Time: 2021-09-08T19:01:43.193+0000

• Duration: 16 min 3 sec

• Commit: 258817c

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Total	256

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