Remove the extra slash while loading resources. (#893)

* Remove the extra slash

* Update src/main/java/emu/grasscutter/Configuration.java

Co-authored-by: HotaruYS <105128850+HotaruYS@users.noreply.github.com>

* Update src/main/java/emu/grasscutter/Configuration.java

Co-authored-by: HotaruYS <105128850+HotaruYS@users.noreply.github.com>

* Update src/main/java/emu/grasscutter/Configuration.java

Co-authored-by: HotaruYS <105128850+HotaruYS@users.noreply.github.com>

* Import java.nio.file.Paths to use Paths.get

* Mark fields as private to prevent use *FOLDER directly

* Remove unnecessary slash

Co-authored-by: HotaruYS <105128850+HotaruYS@users.noreply.github.com>