runtime(optwin): Fix for 'splitkeep' option (#12974)

'spk' was used as a boolean, rather than a string option.

Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/optwin.vim

@@ -1,7 +1,7 @@
 " These commands create the option window.
 "
 " Maintainer:	The Vim Project <https://github.com/vim/vim>
-" Last Change:	2023 Aug 10
+" Last Change:	2023 Aug 31
 " Former Maintainer:	Bram Moolenaar <Bram@vim.org>
 
 " If there already is an option window, jump to that one.
@@ -518,7 +518,7 @@ call <SID>OptionG("swb", &swb)
 call <SID>AddOption("splitbelow", gettext("a new window is put below the current one"))
 call <SID>BinOptionG("sb", &sb)
 call <SID>AddOption("splitkeep", gettext("determines scroll behavior for split windows"))
-call <SID>BinOptionG("spk", &spk)
+call <SID>OptionG("spk", &spk)
 call <SID>AddOption("splitright", gettext("a new window is put right of the current one"))
 call <SID>BinOptionG("spr", &spr)
 call <SID>AddOption("scrollbind", gettext("this window scrolls together with other bound windows"))