config: fix vinyl.write_threads default (2 -> 4)

box.cfg.vinyl_write_threads is 4 by default. Let's set the same value in
the declarative config as the default.

It seems, it was set to 2 by mistake.

Part of #8861

NO_DOC=bugfix
NO_CHANGELOG=the option is introduced after 3.0.0-alpha1 and there were
             no (pre)releases after that