DOCS-6027 fix encryption key command

Author: kay-kim

source/release-notes/3.1-dev-series-configure-encryption.txt

@@ -165,7 +165,7 @@ accessible by the owner of the :program:`mongod` process.
 
    .. code-block:: sh
 
-      openssl rand -base64 32 | head -c 32 > mongodb-keyfile
+      openssl rand -base64 32 > mongodb-keyfile
 
 #. Update the file permissions.