man: fix no-auto-default state dir in NetworkManager.conf manual

Quote from `man NetworkManager.conf`:

  When the default wired connection is deleted or saved to a new
  persistent connection by a plugin, the device is added to a list in the
  file /run/NetworkManager/no-auto-default.state to prevent creating
  the default connection for that device again.

/run is obviously wrong. Fix it.
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