bpo-41572: Fix grammar in BaseTransport.close docstring (GH-21914)

Fix grammar in BaseTransport.close docstring.

https://bugs.python.org/issue41572

Signed-off-by: Cleber Rosa <crosa@redhat.com>

Author: clebergnu

Lib/asyncio/transports.py

@@ -29,8 +29,8 @@ def close(self):
 
         Buffered data will be flushed asynchronously.  No more data
         will be received.  After all buffered data is flushed, the
-        protocol's connection_lost() method will (eventually) called
-        with None as its argument.
+        protocol's connection_lost() method will (eventually) be
+        called with None as its argument.
         """
         raise NotImplementedError