docs: correct default query encoding in README examples

[Likio3000]

The README shows commas and dollar signs unencoded when the examples use default query encoding. Query.encode() defaults to dont_quote='', and furl.tostr() passes the same default through, so those examples should contain %2C and %24.

Correct five expected-output lines and clarify that query_dont_quote='$' affects the returned string without changing the defaults used by f.url. The explicit opt-out example keeps its unencoded dollar sign.

Validation: statically reviewed create_quote_fn, Query.encode, Query.__str__, furl.tostr, and the url property, together with the existing test_encode and test_tostr assertions. Verified original Git blob and diff scope. No furl invocation, imports, project tests or documentation build were run; this changes only README text.

Prepared with AI assistance (OpenAI Codex), with source and diff review.