fixing naming in docs to match the actual defention in apiState

Author: Marvin corro

docs/rtk-query/api/created-api/hooks.mdx

@@ -335,7 +335,7 @@ type UseMutationResult<T> = {
   data?: T // Returned result if present
   error?: unknown // Error result if present
   endpointName?: string // The name of the given endpoint for the mutation
-  fulfilledTimestamp?: number // Timestamp for when the mutation was completed
+  fulfilledTimeStamp?: number // Timestamp for when the mutation was completed
 
   // Derived request status booleans
   isUninitialized: boolean // Mutation has not been fired yet