chore(algebra/ring): use curly brackets for ring_hom where possible

[ChrisHughes24]

TO CONTRIBUTORS:

Make sure you have:

• reviewed and applied the coding style: coding, naming
• reviewed and applied the documentation requirements
• for tactics:
  • added or adapted documentation in tactics.md
  • write an example of use of the new feature in tactics.lean
• make sure definitions and lemmas are put in the right files
• make sure definitions and lemmas are not redundant

If this PR is related to a discussion on Zulip, please include a link in the discussion.

For reviewers: code review check list

[rwbarton]

Let's try to document why we are doing this somewhere, because using implicit arguments like this is not an obvious choice. I suggest writing a Note

/- Note [instances as implicit arguments]
...  -- try to discuss what goes wrong with [] arguments, or why it leads to wasted effort by Lean in specific examples
-/

Specializing to ring or semiring is fine. Then we can also refer to the note in ring_theory.algebra or wherever else we want to make a similar change.