doc: update diagnostics channel request headers type change (#2925)

Author: jessezhang91

docs/docs/api/DiagnosticsChannel.md

@@ -19,9 +19,9 @@ diagnosticsChannel.channel('undici:request:create').subscribe(({ request }) => {
   console.log('completed', request.completed)
   console.log('method', request.method)
   console.log('path', request.path)
-  console.log('headers') // raw text, e.g: 'bar: bar\r\n'
+  console.log('headers') // array of strings, e.g: ['foo', 'bar']
   request.addHeader('hello', 'world')
-  console.log('headers', request.headers) // e.g. 'bar: bar\r\nhello: world\r\n'
+  console.log('headers', request.headers) // e.g. ['foo', 'bar', 'hello', 'world']
 })
 ```