Update README.md about the ^w -> ^s

Now the keybinding for saving files is ^s, as described
on the wishlist issue #1

README.md

@@ -43,7 +43,7 @@ To enable logging, use the parameter `-l <logfile>`. Two files will be written:
 
 ## Shortcuts
 
-- `^w` Save
+- `^s` Save
 - `^q` Quit
 
 ## Caveats