Update count.md

Author: 38elements

doc/api/core/operators/count.md

@@ -1,13 +1,14 @@
-### `Rx.Observable.prototype.count([predicate])`
+### `Rx.Observable.prototype.count([predicate], [thisArg])`
 [Ⓢ](https://github.com/Reactive-Extensions/RxJS/blob/master/src/core/linq/observable/count.js "View in source")
 
 Returns an observable sequence containing a value that represents how many elements in the specified observable sequence satisfy a condition if provided, else the count of items.
 
 #### Arguments
-1. `[predicate]` *(`Any`)*: A function to test each element for a condition.  The callback is called with the following information:
+1. `[predicate]` *(`Function`)*: A function to test each element for a condition.  The callback is called with the following information:
     1. the value of the element
     2. the index of the element
     3. the Observable object being subscribed
+2. `[thisArg]` *(`Any`)*: Object to use as `this` when executing `predicate`.
 
 #### Returns
 *(`Observable`)*: An observable sequence containing a single element with a number that represents how many elements in the input sequence satisfy the condition in the predicate function if provided, else the count of items in the sequence.