doc: fix typo in synchronous randomBytes example

The string template was closed after `${buf.length}`
causing a syntax error within the example.

PR-URL: #5781
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/crypto.markdown

@@ -1243,7 +1243,7 @@ there is a problem generating the bytes.
 // Synchronous
 const buf = crypto.randomBytes(256);
 console.log(
-  `${buf.length}` bytes of random data: ${buf.toString('hex')});
+  `${buf.length} bytes of random data: ${buf.toString('hex')}`);
 ```
 
 The `crypto.randomBytes()` method will block until there is sufficient entropy.