doc: HTTPS client's options

doc/api/https.markdown

@@ -86,6 +86,7 @@ Example:
       path: '/',
       method: 'GET',
       key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
+      cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem')
     };
     options.agent = new https.Agent(options);
 
@@ -103,6 +104,7 @@ Example:
       path: '/',
       method: 'GET',
       key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
+      cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem'),
       agent: false
     };