src: fixes misplaced comment

In e26622b, a comment was incorrectly moved from the code
it was describing.

Fixes: https://github.com/nodejs/node/issues/8856
PR-URL: https://github.com/nodejs/node/pull/8860
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/_stream_transform.js

@@ -94,7 +94,6 @@ function Transform(options) {
 
   this._transformState = new TransformState(this);
 
-  // when the writable side finishes, then flush out anything remaining.
   var stream = this;
 
   // start out asking for a readable event once data is transformed.
@@ -113,6 +112,7 @@ function Transform(options) {
       this._flush = options.flush;
   }
 
+  // When the writable side finishes, then flush out anything remaining.
   this.once('prefinish', function() {
     if (typeof this._flush === 'function')
       this._flush(function(er, data) {