fix: Add changes for upload sourcemaps snippet

[david-luna]

Description

The nodejs snippet provided contains an error on how to pass the file content to the request method. This PR fixes that.

Documentation sets edited in this PR

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• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

N/A

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

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• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)
• This PR needs to be ported to another doc set:
  • Port to stateful docs: <link to PR or tracking issue>
  • Port to serverless docs: <link to PR or tracking issue>

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[colleenmcginnis]

Thanks! Feel free to merge anytime.

Note: I added the backport-8.16 and backport-8.17 as the two most recent maintained versions assuming this change is compatible with those versions.