system/vl: Update description for input grab key

Input grab key should be Ctrl-Alt-g, not just Ctrl-Alt.

Fixes: f8d2c93 ("sdl: use ctrl-alt-g as grab hotkey")
Signed-off-by: Tianlan Zhou <bobby825@126.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

system/vl.c

@@ -891,7 +891,7 @@ static void help(int exitcode)
     printf("\nDuring emulation, the following keys are useful:\n"
            "ctrl-alt-f      toggle full screen\n"
            "ctrl-alt-n      switch to virtual console 'n'\n"
-           "ctrl-alt        toggle mouse and keyboard grab\n"
+           "ctrl-alt-g      toggle mouse and keyboard grab\n"
            "\n"
            "When using -nographic, press 'ctrl-a h' to get some help.\n"
            "\n"