[PATCH] Doc: Fix missing func_signature role in pg_get_tablespace_ddl entry

Hi,

The second function signature of pg_get_tablespace_ddl()
in the "Get Object DDL Functions" table is missing
the func_signature role attribute on its <para> tag.
This causes inconsistent indentation compared to
the first signature in the rendered documentation.

Other functions with multiple signatures, such as current_schema,
use <para role="func_signature"> for each signature entry.
The attached patch fixes this by adding the missing role attribute.

Regards,
Tatsuya Kawata