[ruff] Fix non-triggering example for if-key-in-dict-del (RUF051)

[nikolauspschuetz]

Summary

The if-key-in-dict-del (RUF051) rule docstring's ## Example does not actually trigger the rule.

RUF051 only fires when the dictionary is known to be of type dict (typing::is_known_to_be_of_type_dict). The example used a bare, unbound dictionary, whose type can't be inferred — so the snippet presented as triggering is in fact clean:

$ printf 'if key in dictionary:\n    del dictionary[key]\n' | ruff check --select RUF051 -
All checks passed!

Change

Bind dictionary = {} in both code blocks so the example triggers (and the "Use instead" stays parallel). Docstring-only; no behavior change.

# Example (now triggers)
dictionary = {}
if key in dictionary:
    del dictionary[key]

# Use instead
dictionary = {}
dictionary.pop(key, None)

This matches the convention already used by the sibling dict-type-gated rule if-else-block-instead-of-dict-get (SIM401), whose docstring example likewise binds foo = {}.

Test plan

• Verified the old example is clean and the new one triggers:

  $ printf 'dictionary = {}\nif key in dictionary:\n    del dictionary[key]\n' | ruff check --select RUF051 -
  RUF051 [*] Use `pop` instead of `key in dict` followed by `del dict[key]`

• cargo dev generate-docs regenerates docs/rules/if-key-in-dict-del.md cleanly.
• scripts/check_docs_formatted.py → All docs are formatted correctly.

[ntBre]

Thanks! This fixes RUF051 for the sake of #18972.