fixed warning: unused std::result::Result

```
warning: unused `std::result::Result` that must be used
```

I forgot `?` for the error propagation

src/playlist.rs

@@ -623,7 +623,7 @@ impl MediaSegment {
         if let Some(ref v) = self.title {
             writeln!(w, "{}", v)?;
         } else {
-            write!(w, "\n");
+            write!(w, "\n")?;
         }
 
         writeln!(w, "{}", self.uri)