Corrects capitalization of ARN

Author: gdavison

website/docs/d/appstream_image.html.markdown

@@ -28,7 +28,7 @@ The following arguments are optional:
 * `region` - (Optional) Region where this resource will be [managed](https://docs.aws.amazon.com/general/latest/gr/rande.html#regional-endpoints). Defaults to the Region set in the [provider configuration](https://registry.terraform.io/providers/hashicorp/aws/latest/docs#aws-configuration-reference).
 * `name` - Name of the image being searched for. Cannot be used with name_regex or arn.
 * `name_regex` - Regular expression name of the image being searched for. Cannot be used with arn or name.
-* `arn` - Arn of the image being searched for. Cannot be used with name_regex or name.
+* `arn` - ARN of the image being searched for. Cannot be used with name_regex or name.
 * `type` - The type of image which must be (PUBLIC, PRIVATE, or SHARED).
 * `most_recent` - Boolean that if it is set to true and there are multiple images returned the most recent will be returned. If it is set to false and there are multiple images return the datasource will error.