runtime(help): fix wrong check for existing HelpComplete function

To check for an existing HelpComplete function, exists('*HelpComplete')
should be used, as exists('HelpComplete') still returns 0 after sourcing
the ftplugin.

closes: #20073

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

runtime/ftplugin/help.vim

@@ -24,7 +24,7 @@ if has("conceal")
   setlocal cole=2 cocu=nc
 endif
 
-if !exists('HelpComplete')
+if !exists('*HelpComplete')
   func HelpComplete(findstart, base)
     if a:findstart
       let colnr = col('.') - 1 " Get the column number before the cursor