Fix p0540 filename (#619)

proposals/README.md

@@ -57,6 +57,7 @@ request:
 -   [0447 - Generics terminology](p0447.md)
 -   [0524 - Generics overview](p0524.md)
 -   [0538 - `return` with no argument](p0538.md)
+-   [0540 - Remove `Void`](p0540.md)
 -   [0555 - Operator precedence](p0555.md)
 
 <!-- endproposals -->