Update readme.md

Author: acoumb

src/Umbraco.Forms.Integrations.Crm.ActiveCampaign/readme.md

@@ -13,7 +13,7 @@ Required minimum versions of Umbraco CMS:
 ### Authentication
 
 All requests to the ActiveCampaign API are authenticated by providing
-an API key. The API key is included as an HTTP header called _Api_Token_.
+an API key. The API key is included as an HTTP header called _Api-Token_.
 
 If the configuration is incomplete, the user will receive an error message.