Update arg lists for 2 events in event_callbacks.rst

[vwheeler63]

Purpose

Documentation in ./doc/extdev/event_callbacks.rst showed 2 events with an old (or incomplete) list of arguments in the "Core events overview" section, specifically their leading app argument. This was remedied.

References

• Python/Lib/site-packages/sphinx/domains/std/init.py line 1443 function:

  def warn_missing_reference(
      app: Sphinx,
      domain: Domain,
      node: pending_xref,
  ) -> bool | None:
      ...

• Python/Lib/site-packages/sphinx/ext/intersphinx/_resolve.py line 339 function:

  def missing_reference(
      app: Sphinx, env: BuildEnvironment, node: pending_xref, contnode: TextElement
  ) -> nodes.reference | None:
      """Attempt to resolve a missing reference via intersphinx references."""
      return resolve_reference_detect_inventory(env, node, contnode)