doc: fix typo in streams.md

PR-URL: nodejs/node#12428
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/stream.md

@@ -1108,7 +1108,7 @@ implement streams using JavaScript's prototypal inheritance model.
 
 First, a stream developer would declare a new JavaScript class that extends one
 of the four basic stream classes (`stream.Writable`, `stream.Readable`,
-`stream.Duplex`, or `stream.Transform`), making sure the call the appropriate
+`stream.Duplex`, or `stream.Transform`), making sure they call the appropriate
 parent class constructor:
 
 ```js