fix: change autocmd event from VimEnter to BufEnter

this commit fixes the situation below:
1. open vim without argument(file)
2. :e main.tf
3. didn't load vim-terraform autocmd like :TerraformFmt command

ftplugin/terraform.vim

@@ -59,9 +59,9 @@ endfunction
 
 augroup terraform
   autocmd!
-  autocmd VimEnter *
+  autocmd BufEnter *
         \ command! -nargs=+ -complete=custom,s:commands Terraform execute '!terraform '.<q-args>. ' -no-color'
-  autocmd VimEnter * command! -nargs=0 TerraformFmt call terraform#fmt()
+  autocmd BufEnter * command! -nargs=0 TerraformFmt call terraform#fmt()
   if get(g:, "terraform_fmt_on_save", 1)
     autocmd BufWritePre *.tf call terraform#fmt()
     autocmd BufWritePre *.tfvars call terraform#fmt()