Fix duplicate link references in README

[Jefffrey]

Markdown can't handle multiple reference links with same name. i.e.

Check out [this link].

[this link]: https://github.com/apache/arrow-rs

Now how about [this link].

[this link]: https://github.com/apache/datafusion

The link on Now how about this link will point to arrow-rs instead of datafusion.

Example of how that's affecting current README:

For now I've decided to include the issue number as part of the text so the reference links have unique names.

Alternatives were either slightly change the text, like making the second one:

See [this issue] for more nuances.

Which I didn't like as I feel we may fall into this issue again.

Another alternative would be to inline both:

Check out [this link](https://github.com/apache/arrow-rs).

Now how about [this link](https://github.com/apache/datafusion)

Don't have a particular preference for that or current approach.

[etseidl]

Nice catch! I think in this case including the issue number in the text is the right way to go. But I'd be fine with [text](url) as well if others have a strong preference.

[alamb]

I agree -- this looks great -- thanks @Jefffrey