Fix description for ContentId of Attachment model

not showing example image tag usage

Author: M-Yankov

src/SendGrid/Helpers/Mail/Model/Attachment.cs

@@ -38,7 +38,7 @@ public class Attachment
         public string Disposition { get; set; }
 
         /// <summary>
-        /// Gets or sets a unique id that you specify for the attachment. This is used when the disposition is set to "inline" and the attachment is an image, allowing the file to be displayed within the body of your email. Ex: <img src="cid:ii_139db99fdb5c3704"></img>.
+        /// Gets or sets a unique id that you specify for the attachment. This is used when the disposition is set to "inline" and the attachment is an image, allowing the file to be displayed within the body of your email. Ex: &lt;img src="cid:ii_139db99fdb5c3704"&gt;&lt;/img&gt;.
         /// </summary>
         [JsonProperty(PropertyName = "content_id")]
         public string ContentId { get; set; }