docs: fix ToC file extension for RFC 004. (#7813)

docs/rfc/README.md

@@ -41,7 +41,7 @@ sections.
 - [RFC-001: Storage Engines](./rfc-001-storage-engine.rst)
 - [RFC-002: Interprocess Communication](./rfc-002-ipc-ecosystem.md)
 - [RFC-003: Performance Taxonomy](./rfc-003-performance-questions.md)
-- [RFC-004: E2E Test Framework Enhancements](./rfc-004-e2e-framework.md)
+- [RFC-004: E2E Test Framework Enhancements](./rfc-004-e2e-framework.rst)
 - [RFC-005: Event System](./rfc-005-event-system.rst)
 - [RFC-006: Event Subscription](./rfc-006-event-subscription.md)
 - [RFC-007: Deterministic Proto Byte Serialization](./rfc-007-deterministic-proto-bytes.md)