Updating deprecated docstring Client() class (#3426)

Author: Danarvelini

httpx/_client.py

@@ -620,8 +620,6 @@ class Client(BaseClient):
     * **http2** - *(optional)* A boolean indicating if HTTP/2 support should be
     enabled. Defaults to `False`.
     * **proxy** - *(optional)* A proxy URL where all the traffic should be routed.
-    * **proxies** - *(optional)* A dictionary mapping proxy keys to proxy
-    URLs.
     * **timeout** - *(optional)* The timeout configuration to use when sending
     requests.
     * **limits** - *(optional)* The limits configuration to use.