attribute reference updated with pool_arn

[sumitmenaria]

data source aws_ec2_coip_pool documentation updated with pool_arn attribute

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Closes hashicorp#16426

Release note for CHANGELOG:

datasource/aws_ec2_coip_pool added pool_arn attribute

Output from acceptance testing:

$ make testacc TEST=./aws TESTARGS='-run=TestAccDataSourceAwsEc2CoipPool_Id'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -count 1 -parallel 20 -run=TestAccDataSourceAwsEc2CoipPool_Id -timeout 120m
=== RUN   TestAccDataSourceAwsEc2CoipPool_Id
=== PAUSE TestAccDataSourceAwsEc2CoipPool_Id
=== CONT  TestAccDataSourceAwsEc2CoipPool_Id
--- PASS: TestAccDataSourceAwsEc2CoipPool_Id (34.21s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	34.272s

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