fix: change 'show' tactic to work as documented

[Rob23oba]

This PR changes the behavior of the 'show' tactic to work as documented, i.e. unify with the first goal possible and make that the main goal.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 141e519009fc74d444cb8a32b55daf3c2d192bbd --onto ca0d8226192e7c0cdcc71d6322c3226ad4f73f30. You can force Mathlib CI using the force-mathlib-ci label. (2025-03-08 11:41:46)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase b9f8a859e7f354f026de7c910e17ce29e23678f9 --onto ca0d8226192e7c0cdcc71d6322c3226ad4f73f30. You can force Mathlib CI using the force-mathlib-ci label. (2025-03-08 13:41:28)

[nomeata]

Thanks! Just wants to let you know that this is a change we want. Didn't look at the changes yet, since it's still a draft. And maybe I'll leave it to @kmill to review when he gets to it.