Update vcpkg rev in docs
Update vcpkg rev in docs

Update the rev given for vcpkg in the README to point at the same rev
used for rust-sdl2 testing.

Rust-SDL2/rust-sdl2,Cobrand/rust-sdl2,Cobrand/rust-sdl2,Rust-SDL2/rust-sdl2,Rust-SDL2/rust-sdl2,Cobrand/rust-sdl2