doc: don't use useless constructors in stream.md

PR-URL: #13145
Refs: http://eslint.org/docs/rules/no-useless-constructor
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/stream.md

@@ -1128,6 +1128,7 @@ const Writable = require('stream').Writable;
 class MyWritable extends Writable {
   constructor(options) {
     super(options);
+    // ...
   }
 }
 ```
@@ -1257,6 +1258,7 @@ class MyWritable extends Writable {
   constructor(options) {
     // Calls the stream.Writable() constructor
     super(options);
+    // ...
   }
 }
 ```
@@ -1393,6 +1395,7 @@ const Writable = require('stream').Writable;
 class MyWritable extends Writable {
   constructor(options) {
     super(options);
+    // ...
   }
 
   _write(chunk, encoding, callback) {
@@ -1435,6 +1438,7 @@ class MyReadable extends Readable {
   constructor(options) {
     // Calls the stream.Readable(options) constructor
     super(options);
+    // ...
   }
 }
 ```
@@ -1648,6 +1652,7 @@ const Duplex = require('stream').Duplex;
 class MyDuplex extends Duplex {
   constructor(options) {
     super(options);
+    // ...
   }
 }
 ```
@@ -1803,6 +1808,7 @@ const Transform = require('stream').Transform;
 class MyTransform extends Transform {
   constructor(options) {
     super(options);
+    // ...
   }
 }
 ```