runtime(just): Correct filetype detection pattern and style (#13284)

See https://github.com/vim/vim/pull/13271#discussion_r1347279686 and
https://github.com/NoahTheDuke/vim-just/blob/main/ftdetect/just.vim.

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/filetype.vim b/runtime/filetype.vim
index f304963..3e7036c 100644
--- a/runtime/filetype.vim
+++ b/runtime/filetype.vim
@@ -1094,7 +1094,7 @@
 au BufNewFile,BufRead *.jl			setf julia
 
 " Just
-autocmd BufNewFile,BufRead *[jJ]ustfile,.justfile,*.just setfiletype just
+au BufNewFile,BufRead [jJ]ustfile,.justfile,*.just setf just
 
 " KDL
 au BufNewFile,BufRead *.kdl			setf kdl