Fix warning in ConcurrentOperationExecutor.kt

Prior to this commit, the following warning was emitted in the build.

w: file:///<path>/spring-framework/framework-docs/src/main/kotlin/org/springframework/docs/core/aop/ataspectj/aopataspectjexample/ConcurrentOperationExecutor.kt:57:29 Unnecessary non-null assertion (!!) on a non-null receiver of type 'PessimisticLockingFailureException'.

framework-docs/src/main/kotlin/org/springframework/docs/core/aop/ataspectj/aopataspectjexample/ConcurrentOperationExecutor.kt

@@ -54,6 +54,6 @@ class ConcurrentOperationExecutor : Ordered {
 				lockFailureException = ex
 			}
 		} while (numAttempts <= this.maxRetries)
-		throw lockFailureException!!
+		throw lockFailureException
 	}
 } // end::snippet[]