sync: change description arrow

[ee7]

With commit aa23b4d (#301), configlet sync writes a fuller
description value to tests.toml, using a → arrow to indicate
nesting.

However, the → symbol (Rightwards Arrow U+2192) doesn't always render
well. A symbol like 🡲 (Wide-Headed Rightwards Medium Barb Arrow
U+1F872) can render better in the good case, but I easily found places
where it renders worse in the bad case.

Let's just go with -> for now.

There's an argument that -> is more likely to appear in a
description in a canonical-data.json, but that'd still look strange
even if we use a different arrow to indicate nesting.

---

Follow-up from #301 (review):

Okay, I'll merge then. I'll make a follow-up PR later for the delimiter bikeshedding.

It's just that I want to avoid a mass PR that updates every → to e.g. -> on every track. And I think that we should consider making a mass PR that adds reimplements, the fuller description, and the new comment at the top of every tests.toml. This would better explain how tests.toml works, and prevent noisy diffs in the first PR for every track exercise that touches the corresponding tests.toml.

So if we do want such a mass PR, I think we should settle the delimiter decision and the tests.toml comment wording first. We might also want to settle these things before creating the next configlet release.

---

Some arrows:
→
->
⟶
➙
➔

🡢
🡪
🡲
🡺
🢂

https://unicode-table.com/en/sets/arrow-symbols/
http://xahlee.info/comp/unicode_arrows.html