pg_create_logical_replication_slot argument incongruency

Hello,

The function `pg_create_logical_replication_slot()` is documented to have
a `two_phase` argument(note the underscore), but the function instead
requires `twophase`.

```
\df pg_catalog.pg_create_logical_replication_slot
List of functions
-[ RECORD 1
]-------+---------------------------------------------------------------------------------------------------------------------------------

Schema | pg_catalog
Name | pg_create_logical_replication_slot
Result data type | record
Argument data types | slot_name name, plugin name, temporary boolean
DEFAULT false, twophase boolean DEFAULT false, OUT slot_name name, OUT lsn
pg_lsn
Type | func
```

This was introduced in commit 19890a06.

IMHO we should use the documented argument name `two_phase` and change the
function to accept it.

What do you think?

Please, check the attached patch.

Cheers,
Florin
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