Update usage.rst (#572)

fix hstore example error, change con to conn

Author: xuedong09

docs/usage.rst

@@ -318,9 +318,9 @@ be registered on a connection using :meth:`Connection.set_builtin_type_codec()
     async def run():
         conn = await asyncpg.connect()
         # Assuming the hstore extension exists in the public schema.
-        await con.set_builtin_type_codec(
+        await conn.set_builtin_type_codec(
             'hstore', codec_name='pg_contrib.hstore')
-        result = await con.fetchval("SELECT 'a=>1,b=>2'::hstore")
+        result = await conn.fetchval("SELECT 'a=>1,b=>2'::hstore")
         assert result == {'a': 1, 'b': 2}
 
     asyncio.get_event_loop().run_until_complete(run())