fix(tactic/delta_instance): beta reduce type of instance (#6396)
fix(tactic/delta_instance): beta reduce type of instance (#6396)

The delta derive handler was creating instances that weren't beta reduced, which isn't a problem for type class inference but can be unexpected. @gebner fixed the line in doc-gen that assumed beta reduction, but we should also produce the expected instance in the first place.

Zulip: https://leanprover.zulipchat.com/#narrow/stream/113488-general/topic/doc-gen.20is.20failing



Co-authored-by: Rob Lewis <597e2d195b18d1f0b2b6530cee088a0ab0ed3ce2@gmail.com>

leanprover-community/mathlib,leanprover-community/mathlib