doc: add missing commas in cluster docs

PR-URL: nodejs#41865
Reviewed-By: Derek Lewis <DerekNonGeneric@inf.is>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Mestery <mestery@protonmail.com>

doc/api/cluster.md

@@ -635,10 +635,10 @@ changes:
 
 Send a message to a worker or primary, optionally with a handle.
 
-In the primary this sends a message to a specific worker. It is identical to
+In the primary, this sends a message to a specific worker. It is identical to
 [`ChildProcess.send()`][].
 
-In a worker this sends a message to the primary. It is identical to
+In a worker, this sends a message to the primary. It is identical to
 `process.send()`.
 
 This example will echo back all messages from the primary:
@@ -741,7 +741,7 @@ added: v0.7.0
 * `address` {Object}
 
 After calling `listen()` from a worker, when the `'listening'` event is emitted
-on the server a `'listening'` event will also be emitted on `cluster` in the
+on the server, a `'listening'` event will also be emitted on `cluster` in the
 primary.
 
 The event handler is executed with two arguments, the `worker` contains the