doc: fix code type of markdowns

1. correct code type in addons.markdown
2. add missed code type in crypto.markdown

PR-URL: https://github.com/nodejs/node/pull/4858
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>

doc/api/addons.markdown

@@ -364,7 +364,7 @@ adding the function as a property of `exports`.
 
 To test it, run the following JavaScript:
 
-```cpp
+```js
 // test.js
 const addon = require('./build/Release/addon');
 

doc/api/crypto.markdown

@@ -235,7 +235,7 @@ decipher.end();
 
 Example: Using `Decipher` and piped streams:
 
-```
+```js
 const crypto = require('crypto');
 const fs = require('fs');
 const decipher = crypto.createDecipher('aes192', 'a password');