Update error message regarding #447

[amrojjeh]

fn getWriter() anytype {
    return std.io.getStdOut().writer();
}

When I compile the code shown above, I get the error message: error: TODO implement inferred return types https://github.com/ziglang/zig/issues/447

Seeing that #447 has been closed, it seems fair that the error should be updated to reflect the change.