grub2: fix old kernel param delimiter in speech synth entry

ref: debian bug #775128

done in live-build auto-generated menu entries back in v5.0a3 for syslinux
(ba6b9adeff634845a9ab7cbd5218257a7aa254ab). was missing for grub entries
until a33938f8d47f23c23335ecf671b1db9afcd0b726 and
aadf8c4cacecc4664412c6bbc7b8e039e1dd772a.

kali-config/common/hooks/live/accessibility-menu.binary

@@ -10,6 +10,6 @@ label installspk
 	menu label Install with ^speech synthesis
 	linux /install/gtk/vmlinuz
 	initrd /install/gtk/initrd.gz
-	append video=vesa:ywrap,mtrr vga=788 speakup.synth=soft -- quiet
+	append video=vesa:ywrap,mtrr vga=788 speakup.synth=soft --- quiet
 END