doc: fix incorrect syntax in examples

The cluster docs had a period instead of a semicolon at the end of two
lines.

PR-URL: https://github.com/nodejs/node/pull/6463
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Jackson Tian <shvyo1987@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/cluster.md

@@ -305,7 +305,7 @@ this value.
 ```js
 cluster.on('exit', (worker, code, signal) => {
   if (worker.exitedAfterDisconnect === true) {
-    console.log('Oh, it was just voluntary\' – no need to worry').
+    console.log('Oh, it was just voluntary – no need to worry');
   }
 });
 
@@ -408,7 +408,7 @@ this value.
 ```js
 cluster.on('exit', (worker, code, signal) => {
   if (worker.suicide === true) {
-    console.log('Oh, it was just voluntary\' – no need to worry').
+    console.log('Oh, it was just voluntary – no need to worry');
   }
 });