Streamline and clean up spacing in TTYPort example

Let's have more compact example and don't let an empty line slip in the
rendered example due to the empty lines before and after using 'master'
and 'slave' not being merged.

src/posix/tty.rs

@@ -51,14 +51,10 @@ fn close(fd: RawFd) {
 /// use serialport::{TTYPort, SerialPort};
 ///
 /// let (mut master, mut slave) = TTYPort::pair().expect("Unable to create ptty pair");
-///
 /// # let _ = &mut master;
 /// # let _ = &mut slave;
-///
 /// // ... elsewhere
-///
 /// let mut port = TTYPort::open(&serialport::new(slave.name().unwrap(), 0)).expect("Unable to open");
-///
 /// # let _ = &mut port;
 /// ```
 #[derive(Debug)]