Corrected comment about root being global on the server

The corresponding comment in the Backbone source was corrected on this point about 9 months ago (see: jashkenas/backbone#1751)

underscore.js

@@ -8,7 +8,7 @@
   // Baseline setup
   // --------------
 
-  // Establish the root object, `window` in the browser, or `global` on the server.
+  // Establish the root object, `window` in the browser, or `exports` on the server.
   var root = this;
 
   // Save the previous value of the `_` variable.