Docs: Non-crossplatform assertion in stdio example

In the stdio.cpp example there is the following assertion: `assert(!ec || (ec == asio::error::eof));` after reading from the pipe. On Windows it would fire because `ec` would be `asio::error::broken_pipe`. As far as I understand, if the process ended the writing part of the pipe has been closed so the error won't be `eof`. See [MSDN](https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-readfile#pipes) and also an [ASIO issue](https://github.com/chriskohlhoff/asio/issues/1442).