psql: add tab completion for subscription wal_receiver_timeout

Hi,

While testing "[fb80f388f] Add per-subscription wal_receiver_timeout setting", I noticed that wal_receiver_timeout is missing from tab completion for both CREATE SUBSCRIPTION WITH and ALTER SUBSCRIPTION SET:
```
evantest=# create subscription ss with (
BINARY                 CREATE_SLOT            FAILOVER               PASSWORD_REQUIRED      SLOT_NAME              TWO_PHASE
CONNECT                DISABLE_ON_ERROR       MAX_RETENTION_DURATION RETAIN_DEAD_TUPLES     STREAMING
COPY_DATA              ENABLED                ORIGIN                 RUN_AS_OWNER           SYNCHRONOUS_COMMIT
evantest=# alter subscription ss set (
BINARY                 FAILOVER               ORIGIN                 RETAIN_DEAD_TUPLES     SLOT_NAME              SYNCHRONOUS_COMMIT
DISABLE_ON_ERROR       MAX_RETENTION_DURATION PASSWORD_REQUIRED      RUN_AS_OWNER           STREAMING              TWO_PHASE
```

This is a tiny patch to fix that.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/