r/aws_guardduty_invite_accepter: Fix BadRequestException on create

[ewbankkit]

Description

Fixes buggy logic introduced during AWS SDK for Go v2 migration (#38400).

Relations

Closes #38833.

Output from Acceptance Testing

% AWS_GUARDDUTY_MEMBER_ACCOUNT_ID=... AWS_GUARDDUTY_MEMBER_EMAIL=... make testacc TESTARGS='-run=TestAccGuardDuty_serial/InviteAccepter' PKG=guardduty
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go1.23.0 test ./internal/service/guardduty/... -v -count 1 -parallel 20  -run=TestAccGuardDuty_serial/InviteAccepter -timeout 360m
=== RUN   TestAccGuardDuty_serial
=== PAUSE TestAccGuardDuty_serial
=== CONT  TestAccGuardDuty_serial
=== RUN   TestAccGuardDuty_serial/InviteAccepter
=== RUN   TestAccGuardDuty_serial/InviteAccepter/basic
--- PASS: TestAccGuardDuty_serial (21.96s)
    --- PASS: TestAccGuardDuty_serial/InviteAccepter (21.96s)
        --- PASS: TestAccGuardDuty_serial/InviteAccepter/basic (21.96s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/guardduty	26.970s

[github-actions]

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[jar-b]

LGTM 🚀

[github-actions]

This functionality has been released in v5.65.0 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!