Improve default for `spring.diffsync.path`

[jayanderson]

https://github.com/spring-projects/spring-sync/blob/master/spring-sync-web/src/main/java/org/springframework/sync/diffsync/web/DiffSyncController.java#L60

As it currently stands if you also attempt to include static resources at /** it will conflict with /{resource}. If the default were diffsync or something similar then we wouldn't run into this conflict. Alternately it could provide no default and require the user of this class to ensure that property is set to something of their choosing.

In any case we ran into this issue and it took a little while to track down what was preventing our static resource from being requested.