update read-concern option syntax

Author: kay-kim

source/includes/fact-readConcern-syntax.rst

@@ -4,11 +4,4 @@ The readConcern option has the following syntax:
 
 .. code-block:: javascript
 
-   readConcern: { level: <value>,
-                  afterClusterTime: <Timestamp> }
-
-.. important::
-
-   Do not manually set the ``afterClusterTime``. MongoDB drivers set
-   this value automatically for operations associated with
-   :ref:`causally consistent sessions <causal-consistency>`.
+   readConcern: { level: <value> }