chore(ssm): fix docs of dataType default value (aws#19739)

This PR fix jsdoc of `dataType` default value.
As described in the following document, `dataType` default value is `text`.
https://docs.aws.amazon.com/ja_jp/AWSCloudFormation/latest/UserGuide/aws-resource-ssm-parameter.html#cfn-ssm-parameter-datatype

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packages/@aws-cdk/aws-ssm/lib/parameter.ts

@@ -141,7 +141,7 @@ export interface StringParameterProps extends ParameterOptions {
   /**
    * The data type of the parameter, such as `text` or `aws:ec2:image`.
    *
-   * @default - undefined
+   * @default ParameterDataType.TEXT
    */
   readonly dataType?: ParameterDataType;
 }