src: fix spelling mistake

Signed-off-by: Trevor Norris <trev.norris@gmail.com>

lib/timers.js

@@ -461,7 +461,7 @@ exports.clearImmediate = function(immediate) {
 };
 
 
-// Internal APIs that need timeouts should use timers._unrefActive isntead of
+// Internal APIs that need timeouts should use timers._unrefActive instead of
 // timers.active as internal timeouts shouldn't hold the loop open
 
 var unrefList, unrefTimer;