typo in aws.rds.write_iops field in processor

[aspacca]

Bug

What does this PR do?

use the proper to value aws.rds.write.iops instead of aws.rds.write_iops for rename processor in rds metricset

Why is it important?

Fields were renamed in #28350.
A typo in the rename processor for aws.rds.write.iops slipped through CR.
The processor is ingesting the data in the wrong field.

Checklist

• My code follows the style guidelines of this project
  - [ ] I have commented my code, particularly in hard-to-understand areas
  - [ ] I have made corresponding changes to the documentation
  - [ ] I have made corresponding change to the default configuration files
  - [ ] I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• [ ]

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[mergify]

This pull request does not have a backport label. Could you fix it @aspacca? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

Pinging @elastic/integrations (Team:Platforms)

[elasticmachine]

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Build stats

• Start Time: 2021-11-15T12:16:39.663+0000

• Duration: 79 min 8 sec

• Commit: ef9c881

Test stats 🧪

Test	Results
Failed	0
Passed	2661
Skipped	240
Total	2901

💚 Flaky test report

Tests succeeded.

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