Fix .first() usage

Signed-off-by: Maxim Zhiburt <zhiburt@gmail.com>

src/session/sync_session.rs

@@ -146,7 +146,7 @@ impl<P, S: Read + NonBlocking> Session<P, S> {
     /// // wait to guarantee that check will successed (most likely)
     /// std::thread::sleep(std::time::Duration::from_secs(1));
     /// let m = p.check(expectrl::Regex("\\d+")).unwrap();
-    /// assert_eq!(m.first(), b"123");
+    /// assert_eq!(m.matches()[0], b"123");
     /// ```
     pub fn check<E: Needle>(&mut self, needle: E) -> Result<Found, Error> {
         let eof = self.stream.read_available()?;