fix(codegen): fix typo to codegen filename ext (#279)

Author: kggilmer

codegen/smithy-aws-kotlin-codegen/src/main/kotlin/aws/sdk/kotlin/codegen/customization/s3/S3ErrorMetadataIntegration.kt

@@ -25,7 +25,7 @@ class S3ErrorMetadataIntegration : KotlinIntegration {
 
     override fun writeAdditionalFiles(ctx: CodegenContext, delegator: KotlinDelegator) {
 
-        delegator.useFileWriter("S3ErrorMetadata", "${ctx.settings.pkg.name}.model") { writer ->
+        delegator.useFileWriter("S3ErrorMetadata.kt", "${ctx.settings.pkg.name}.model") { writer ->
             writer.addImport(AwsRuntimeTypes.Core.AwsErrorMetadata)
             writer.addImport(RuntimeTypes.Utils.AttributeKey)