Update OpenAPISpec.yaml

Author: kculala

openapispecs/commerceextensions/OpenAPISpec.yaml

@@ -157,7 +157,7 @@ tags:
 
       ### Immutable
 
-      When [creating](/docs/api/commerce-extensions/create-a-custom-field#request) a Custom Field can be configured to be `immutable`. When set to true, the value of this field can be specified only during POST requests and cannot be modified during PUT requests. By default, this is `false`.
+      When [creating](/docs/api/commerce-extensions/create-a-custom-field#request) a Custom Field it can be configured to be `immutable`. When set to true, the value of this field can be specified only during POST requests and cannot be modified during PUT requests. By default, this is `false`.
 
       sample validation object :