docs: Fix name of GtkFileChooser's Create Folder button

docs: Fix name of GtkFileChooser's Create Folder button

The documentation for GtkFileChooser references the button which allows
the user to create new folders as the New Folder button, although the
label actually says Create Folder.

Update the documentation to say Create Folder instead of New Folder.

https://bugzilla.gnome.org/show_bug.cgi?id=729475