fix missing default value in the doc (#4854)

Fix the missing default value in the document function signature. According to source code [client.py](https://github.com/aio-libs/aiohttp/blob/master/aiohttp/client.py) line 151, the defalut value should be `None`.
aio-libs · Oct 16, 2020 · 23da8fd · 23da8fd
1 parent 6e82116
commit 23da8fd
Showing 1 changed file with 1 addition and 1 deletion.
diff --git a/docs/client_reference.rst b/docs/client_reference.rst
@@ -1504,7 +1504,7 @@ ClientTimeout
 ^^^^^^^^^^^^^
 
 .. class:: ClientTimeout(*, total=None, connect=None, \
-                         sock_connect, sock_read=None)
+                         sock_connect=None, sock_read=None)
 
    A data class for client timeout settings.