doc: remove default parameter value from header

In the docs, we specify the default value of function parameters in the
list below the header. There is one exception where we use default
parameter notation. Change that instance to be like the rest of the docs
instead.

I rather like the default parameter notation, and I get why we didn't
use it to begin with. (The notation didn't exist in JavaScript at the
time.) I wouldn't mind switching to it, but that would result in big
churn. That can be a separate topic of discussion. For now, though,
let's get the docs consistent.

PR-URL: https://github.com/nodejs/node/pull/33752
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

doc/api/cluster.md

@@ -415,13 +415,13 @@ if (cluster.isMaster) {
 }
 ```
 
-### `worker.kill([signal='SIGTERM'])`
+### `worker.kill([signal])`
 <!-- YAML
 added: v0.9.12
 -->
 
 * `signal` {string} Name of the kill signal to send to the worker
-  process.
+  process. **Default**: `'SIGTERM'`
 
 This function will kill the worker. In the master, it does this by disconnecting
 the `worker.process`, and once disconnected, killing with `signal`. In the