Update average.md

Author: 38elements

doc/api/core/operators/average.md

@@ -1,10 +1,11 @@
-### `Rx.Observable.prototype.average([selector])`
+### `Rx.Observable.prototype.average([selector], [thisArg])`
 [Ⓢ](https://github.com/Reactive-Extensions/RxJS/blob/master/src/core/linq/observable/average.js "View in source")
 
 Computes the average of an observable sequence of values that are in the sequence or obtained by invoking a transform function on each element of the input sequence if present.
 
 #### Arguments
 1. `[selector]` *(`Function`)*: A transform function to apply to each element.
+2. `[thisArg]` *(`Any`)*: Object to use as `this` when executing `selector`.
 
 #### Returns
 *(`Observable`)*: An observable sequence containing a single element with the average of the sequence of values.