fix: pretty-printing of class abbrev syntax

[grunweg]

This PR makes the pretty-printing of class abbrev syntax match its doc-string. The previous output was missing lots of intermediate spaces.

Found by using the pretty-printer for formatting linting in leanprover-community/mathlib4#30658.

Let me test this change first before asking for review.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Mathlib CI can not be attempted yet, as the nightly-testing-2025-12-23 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-mathlib, Mathlib CI should run now. You can force Mathlib CI using the force-mathlib-ci label. (2026-01-14 17:01:43)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI can not be attempted yet, as the nightly-testing-2025-12-23 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-manual, reference manual CI should run now. You can force reference manual CI using the force-manual-ci label. (2026-01-14 17:01:44)

[nomeata]

Let me test this change first before asking for review.

I recommend to use the draft PR feature whenever you don't want people to look or merge yet

[grunweg]

Let's hope this attempt works: I'd like to get a branch of mathlib, so I can test the pretty-printing there...