doc: fix string interpolation in Stream 'finish'

PR-URL: nodejs/node#12221
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/stream.md

@@ -281,7 +281,7 @@ has been called, and all data has been flushed to the underlying system.
 ```js
 const writer = getWritableStreamSomehow();
 for (var i = 0; i < 100; i ++) {
-  writer.write('hello, #${i}!\n');
+  writer.write(`hello, #${i}!\n`);
 }
 writer.end('This is the end\n');
 writer.on('finish', () => {