Fix PostgreSQL dependency coordinates in reference documentation.

Fixed the group ID in the docs to align with the new artifact coordinates.

Signed-off-by: Reynald Pader <reynaldmpader@gmail.com>
Closes #2163

src/main/antora/modules/ROOT/pages/r2dbc/getting-started.adoc

@@ -19,7 +19,7 @@ Spring Data R2DBC includes direct support for the following databases:
 * https://github.com/r2dbc/r2dbc-mssql[Microsoft SQL Server] (`io.r2dbc:r2dbc-mssql`)
 * https://github.com/asyncer-io/r2dbc-mysql[MySQL] (`io.asyncer:r2dbc-mysql`)
 * https://github.com/jasync-sql/jasync-sql[jasync-sql MySQL] (`com.github.jasync-sql:jasync-r2dbc-mysql`)
-* https://github.com/r2dbc/r2dbc-postgresql[Postgres] (`io.r2dbc:r2dbc-postgresql`)
+* https://github.com/r2dbc/r2dbc-postgresql[Postgres] (`org.postgresql:r2dbc-postgresql`)
 * https://github.com/oracle/oracle-r2dbc[Oracle] (`com.oracle.database.r2dbc:oracle-r2dbc`)
 
 If you use a different database then your application won’t start up.