man.kak: Fix behaviour when called with no params with Posix shells

Seems dash nevers falls back for ${@-$blah}, it does not considers $@
to be empty. Use ${1-$blah} instead as we never have more than one
parameter.

rc/core/man.kak

@@ -64,7 +64,7 @@ define-command -params ..1 \
 If no argument is passed to the command, the selection will be used as page
 The page can be a word, or a word directly followed by a section number between parenthesis, e.g. kak(1)} \
     man %{ %sh{
-    subject=${@-$kak_selection}
+    subject=${1-$kak_selection}
 
     ## The completion suggestions display the page number, strip them if present
     case "${subject}" in