fix typo in Callback Object

Signed-off-by: Mike Ralphson <mike.ralphson@gmail.com>

versions/3.1.0.md

@@ -1844,7 +1844,7 @@ description: object created
 
 A map of possible out-of band callbacks related to the parent operation.
 Each value in the map is a [Path Item Object](#pathItemObject) that describes a set of requests that may be initiated by the API provider and the expected responses.
-The key value used to identify the callback object is an expression, evaluated at runtime, that identifies a URL to use for the callback operation.
+The key value used to identify the path item object is an expression, evaluated at runtime, that identifies a URL to use for the callback operation.
 
 To describe incoming requests from the API provider independent from another API call, use the [`webhooks`](#oasWebhooks) field.