doc: fix indent in tls resumption example

Markdown requires 4-space indent to correctly format code blocks. This
fixes the example so it's correctly presented as code.

PR-URL: https://github.com/nodejs/node/pull/3372
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

doc/api/tls.markdown

@@ -606,14 +606,14 @@ established after addition of event listener.
 
 Here's an example for using TLS session resumption:
 
-  var tlsSessionStore = {};
-  server.on('newSession', function(id, data, cb) {
-    tlsSessionStore[id.toString('hex')] = data;
-    cb();
-  });
-  server.on('resumeSession', function(id, cb) {
-    cb(null, tlsSessionStore[id.toString('hex')] || null);
-  });
+    var tlsSessionStore = {};
+    server.on('newSession', function(id, data, cb) {
+      tlsSessionStore[id.toString('hex')] = data;
+      cb();
+    });
+    server.on('resumeSession', function(id, cb) {
+      cb(null, tlsSessionStore[id.toString('hex')] || null);
+    });
 
 ### Event: 'OCSPRequest'