Unbreak type_inst value in collectd psql view.

There was an error in the `collectd` view where `plugin_inst` was used instead of the correct `type_inst`.

#### Before
```
collectd=# SELECT * FROM collectd LIMIT 2;
  host   | plugin | plugin_inst |   type    |       type_inst       |      identifier         |            tstamp             | name | value
---------+--------+-------------+-----------+-----------------------+-------------------------+-------------------------------+------+-------
 switch0 | snmp   |             | if_octets | GigabitEthernet1_0_22 | switch0/snmp/if_octets- | 2013-06-08 16:06:31.625519+02 | 'rx' |     0
 switch0 | snmp   |             | if_octets | GigabitEthernet1_0_22 | switch0/snmp/if_octets- | 2013-06-08 16:06:31.625519+02 | 'tx' |     0
```
#### After
```
collectd=# SELECT * FROM collectd LIMIT 2;
  host   | plugin | plugin_inst |   type    |       type_inst       |      identifier                              |            tstamp             | name | value
---------+--------+-------------+-----------+-----------------------+----------------------------------------------+-------------------------------+------+-------
 switch0 | snmp   |             | if_octets | GigabitEthernet1_0_22 | switch0/snmp/if_octets-GigabitEthernet1_0_22 | 2013-06-08 16:06:31.625519+02 | 'rx' |     0
 switch0 | snmp   |             | if_octets | GigabitEthernet1_0_22 | switch0/snmp/if_octets-GigabitEthernet1_0_22 | 2013-06-08 16:06:31.625519+02 | 'tx' |     0
```
(look at the `identifier` column)