Update server-session.txt

Corrects an error -- sessions are not available on standalone instances.

Author: xdg

source/reference/server-sessions.txt

@@ -24,8 +24,7 @@ support :ref:`causal-consistency` and :ref:`retryable writes <retryable-writes>`
    Applications use :ref:`client sessions <3.6-client-sessions>` to
    interface with server sessions.
 
-Server sessions are available for standalone :binary:`~bin.mongod`
-instances, replica sets, and sharded clusters. 
+Server sessions are available for replica sets and sharded clusters only. 
 
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