docs: correct how modern_di_fixture resolves in the README API section

[lesnik512]

The README's ## API section said modern_di_fixture resolves a type via
container.resolve and a Provider via container.resolve_provider. It does
neither: factory.py calls container.resolve_dependency(dependency), and the
type-or-provider dispatch happens inside modern-di.

Both methods still exist as public methods on Container in modern-di 3.4.0, so
nothing about the README was broken links or dead API — only the attribution of
which call this package makes. It went stale in 2.1.0, when the inline
isinstance dispatch was replaced by resolve_dependency
(planning/releases/2.1.0.md); the README was not updated alongside it.

The replacement names the one call this package makes and stops describing where
modern-di routes it from there.

Non-goals: the inject/register wording elsewhere in the README and in
factory.py docstrings is left alone — that belongs to the in-flight vocabulary
migration on docs/migrate-off-planning. This change touches lines that branch
does not.

Tiny lane: docs-only, no behaviour change, no change file. AGENTS.md and
architecture/ already describe the delegation correctly, so the README was the
only stale spot.

Verification: just lint-ci and just test-ci both pass (16 tests, 100%
coverage).