vim-patch:a1dc649: runtime(dosbatch): Show %%i as an argument in syntax file

Inside batch files, for-variables must be written as %%i, not %i.

closes: vim/vim#15453

https://github.com/vim/vim/commit/a1dc64956f36cb921d556910c5356bb6b91ba5b9

Co-authored-by: Ken Takata <kentkt@csc.jp>

runtime/syntax/dosbatch.vim

@@ -75,7 +75,7 @@ syn match dosbatchSet		"\s\h\w*[+-]\==\{-1}" contains=dosbatchIdentifier,dosbatc
 
 " Args to bat files and for loops, etc
 syn match dosbatchArgument	"%\(\d\|\*\)"
-syn match dosbatchArgument	"%[a-z]\>"
+syn match dosbatchArgument	"%%[a-z]\>"
 if dosbatch_cmdextversion == 1
   syn match dosbatchArgument	"%\~[fdpnxs]\+\(\($PATH:\)\=[a-z]\|\d\)\>"
 else