Documentation: fix discovery flag for proxy docs

It seems that the -discovery flag used to be -discovery-url. Updated this to use
the currently documented and supported -discovery flag.

Documentation/proxy.md

@@ -5,11 +5,11 @@ etcd can now run as a transparent proxy. Running etcd as a proxy allows for easi
 etcd currently supports two proxy modes: `readwrite` and `readonly`. The default mode is `readwrite`, which forwards both read and write requests to the etcd cluster. A `readonly` etcd proxy only forwards read requests to the etcd cluster, and returns `HTTP 501` to all write requests. 
 
 ### Using an etcd proxy
-To start etcd in proxy mode, you need to provide three flags: `proxy`, `listen-client-urls`, and `initial-cluster` (or `discovery-url`). 
+To start etcd in proxy mode, you need to provide three flags: `proxy`, `listen-client-urls`, and `initial-cluster` (or `discovery`). 
 
 To start a readwrite proxy, set `-proxy on`; To start a readonly proxy, set `-proxy readonly`.
 
-The proxy will be listening on `listen-client-urls` and forward requests to the etcd cluster discovered from in `initial-cluster` or `discovery url`. 
+The proxy will be listening on `listen-client-urls` and forward requests to the etcd cluster discovered from in `initial-cluster` or `discovery` url. 
 
 #### Start an etcd proxy with a static configuration
 To start a proxy that will connect to a statically defined etcd cluster, specify the `initial-cluster` flag:
@@ -18,9 +18,9 @@ etcd -proxy on -listen-client-urls 127.0.0.1:8080 -initial-cluster infra0=http:/
 ```
 
 #### Start an etcd proxy with the discovery service
-If you bootstrap an etcd cluster using the [discovery service][discovery-service], you can also start the proxy with the same `discovery-url`. 
+If you bootstrap an etcd cluster using the [discovery service][discovery-service], you can also start the proxy with the same `discovery`. 
 
-To start a proxy using the discovery service, specify the `discovery-url` flag. The proxy will wait until the etcd cluster defined at the `discovery-url` finishes bootstrapping, and then start to forward the requests. 
+To start a proxy using the discovery service, specify the `discovery` flag. The proxy will wait until the etcd cluster defined at the `discovery` url finishes bootstrapping, and then start to forward the requests. 
 
 ```
 etcd -proxy on -listen-client-urls 127.0.0.1:8080  -discovery https://discovery.etcd.io/3e86b59982e49066c5d813af1c2e2579cbf573de