fix links and update orderbook field names (#750)

services/horizon/internal/docs/reference/resources/orderbook.md

@@ -9,8 +9,8 @@ title: Orderbook
 |--------------|------------------|------------------------------------------------------------------------------------------------------------------------|
 | bids | object     |  Array of {`price_r`, `price`, `amount`} objects (see [offers](./offer.md)).  These represent prices and amounts accounts are willing to buy for the given `selling` and `buying` pair. |
 | asks | object |  Array of {`price_r`, `price`, `amount`} objects (see [offers](./offer.md)).  These represent prices and amounts accounts are willing to sell for the given `selling` and `buying` pair.|
-| selling | [Asset](http://stellar.org/developers/learn/concepts/assets.html) | The Asset this offer wants to sell.|
-| buying | [Asset](http://stellar.org/developers/learn/concepts/assets.html) | The Asset this offer wants to buy.|
+| base | [Asset](http://stellar.org/developers/learn/concepts/assets.html) | The Asset this offer wants to sell.|
+| counter | [Asset](http://stellar.org/developers/learn/concepts/assets.html) | The Asset this offer wants to buy.|
 
 #### Bid Object
 |    Attribute     |  Type  |                                                                                                                                |
@@ -46,4 +46,4 @@ This resource has no links.
 | Resource                 | Type       | Resource URI Template                |
 |--------------------------|------------|--------------------------------------|
 | [Orderbook Details](../orderbook-details.md)       | Single | `/orderbook?{orderbook_params}`       |
-| [Trades for Orderbook](../trades-for-orderbook.md)       | Collection | `/orderbook/trades?{orderbook_params}`       |
+| [Trades](../trades.md)   | Collection | `/trades?{orderbook_params}`       |