doc: remove incorrect synchronous label

fsPromises.rm() is not synchronous.

PR-URL: https://github.com/nodejs/node/pull/35561


Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Ben Coe <bencoe@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Harshitha K P <harshitha014@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>