doc: make first parameter optional in util.getCallSites

`frameCount` is optional because its default value is 10.
And change parameter name from `frameCountOrOptions` to
`frameCount` because it's described as `frameCount` in
below description.

PR-URL: #57387
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jason Zhang <xzha4350@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>

Author: deokjinkim

doc/api/util.md

@@ -450,7 +450,7 @@ util.formatWithOptions({ colors: true }, 'See object %O', { foo: 42 });
 // when printed to a terminal.
 ```
 
-## `util.getCallSites(frameCountOrOptions, [options])`
+## `util.getCallSites([frameCount][, options])`
 
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