Update using-go-and-pgx.rst (#2318)

`v1.18` is released, therefore the note should be updated

Author: teenjuna

docs/guides/using-go-and-pgx.rst

@@ -3,9 +3,7 @@ Using Go and pgx
 ================
 
 .. note::
-   Experimental support for :code:`pgx/v5` was added in v1.17.2. Full support will be
-   included in v1.18.0. Until then, you'll need to pass the :code:`--experimental`
-   flag to :code:`sqlc generate`.
+   :code:`pgx/v5` is supported starting from v1.18.0.
 
 
 pgx is a pure Go driver and toolkit for PostgreSQL. It's become the default