docs: removed false information in copia features

Author: gitmobkab

README.md

@@ -75,7 +75,6 @@ created_at:  past_date()
 - **Realistic data** — built on [Faker](https://faker.readthedocs.io/), 25+ generators out of the box
 - **Relational-aware** — [`ref('table.column')`](https://gitmobkab.github.io/copia/generators/ref) samples from existing rows, so foreign keys just work
 - **Interactive TUI** — write, preview, and insert without leaving your terminal
-- **Extensible** — add your own generators with a single function
 
 ## Installation