docs/system: Update description for input grab key

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

Fixes: f8d2c9369b ("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>

docs/system/keys.rst.inc

@@ -29,7 +29,7 @@ Ctrl-Alt-n
    *3*
       Serial port
 
-Ctrl-Alt
+Ctrl-Alt-g
    Toggle mouse and keyboard grab.
 
 In the virtual consoles, you can use Ctrl-Up, Ctrl-Down, Ctrl-PageUp and