Fix a typo in a comment for Closure.

base/callback.h.pump

 $$ This is a pump file for generating file templates.  Pump is a python
 $$ script that is part of the Google Test suite of utilities.  Description
 $$ can be found here:
 $$
 $$ http://code.google.com/p/googletest/wiki/PumpManual
 $$
 
 $$ See comment for MAX_ARITY in base/bind.h.pump.
 $var MAX_ARITY = 7
 
@@ skipping 409 matching lines @@
   typedef R(*PolymorphicInvoke)(
       internal::BindStateBase*[[]]
 $if ARITY != 0 [[, ]]
 $for ARG , [[typename internal::CallbackParamTraits<A$(ARG)>::ForwardType]]);
 
 };
 
 
 ]]  $$ for ARITY
 
-// Syntactic sugar to make Callbacks<void(void)> easier to declare since it
+// Syntactic sugar to make Callback<void(void)> easier to declare since it
 // will be used in a lot of APIs with delayed execution.
 typedef Callback<void(void)> Closure;
 
 }  // namespace base
 
 #endif  // BASE_CALLBACK_H