Fixup links in README.md

Author: romanz

README.md

@@ -104,8 +104,8 @@ Statistical Programming Meetup (Feb 2021).
 
 Please see [Contribution Guide](CONTRIBUTING.md) for information about contributing to DataFusion.
 
-[arrow]: https://arrow.apache.org/
-[datafusion]: https://github.com/apache/arrow-datafusion
-[flight]: https://arrow.apache.org/blog/2019/10/13/introducing-arrow-flight/
-[flight-sql]: https://arrow.apache.org/blog/2022/02/16/introducing-arrow-flight-sql/
-[ballista-talk]: https://www.youtube.com/watch?v=ZZHQaOap9pQ
+[arrow]:https://arrow.apache.org/
+[datafusion]:https://github.com/apache/arrow-datafusion
+[flight]:https://arrow.apache.org/blog/2019/10/13/introducing-arrow-flight/
+[flight-sql]:https://arrow.apache.org/blog/2022/02/16/introducing-arrow-flight-sql/
+[ballista-talk]:https://www.youtube.com/watch?v=ZZHQaOap9pQ