docs: fix optional params for pubsub

License: MIT
Signed-off-by: Alan Shaw <alan@tableflip.io>

Author: alanshaw

Diff for: README.md

@@ -249,11 +249,11 @@ ipfs config --json API.HTTPHeaders.Access-Control-Allow-Methods "[\"PUT\", \"POS
   - [`ipfs.dht.put(key, value, [callback])`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/DHT.md#dhtput)
 
 - [pubsub](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/PUBSUB.md)
-  - [`ipfs.pubsub.ls(topic, callback)`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/PUBSUB.md#pubsubls)
-  - [`ipfs.pubsub.peers(topic, callback)`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/PUBSUB.md#pubsubpeers)
-  - [`ipfs.pubsub.publish(topic, data, callback)`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/PUBSUB.md#pubsubpublish)
-  - [`ipfs.pubsub.subscribe(topic, handler, options, callback)`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/PUBSUB.md#pubsubsubscribe)
-  - [`ipfs.pubsub.unsubscribe(topic, handler, callback)`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/PUBSUB.md#pubsubunsubscribe)
+  - [`ipfs.pubsub.ls(topic, [callback])`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/PUBSUB.md#pubsubls)
+  - [`ipfs.pubsub.peers(topic, [callback])`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/PUBSUB.md#pubsubpeers)
+  - [`ipfs.pubsub.publish(topic, data, [callback])`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/PUBSUB.md#pubsubpublish)
+  - [`ipfs.pubsub.subscribe(topic, handler, [options], [callback])`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/PUBSUB.md#pubsubsubscribe)
+  - [`ipfs.pubsub.unsubscribe(topic, handler, [callback])`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/PUBSUB.md#pubsubunsubscribe)
 
 - [swarm](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/SWARM.md)
   - [`ipfs.swarm.addrs([callback])`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/SWARM.md#addrs)