Fix gh-853 fo 1.10 feedback: Functions for transaction management

Fixed missing part of a command here (no duplicated strings).

Author: Vladimir I

doc/1.10/book/box/box_txn_management.rst

@@ -75,7 +75,7 @@ Below is a list of all functions for transaction management.
     | :ref:`box.on_rollback()              | Define a trigger that will be   |
     | <box-on_rollback>`                   | activated by ``box.rollback``   |
     +--------------------------------------+---------------------------------+
-    | :ref:`is_in_txn()                    | State whether a transaction is  |
+    | :ref:`box.is_in_txn()                | State whether a transaction is  |
     | <box-is_in_txn>`                     | in progress                     |
     +--------------------------------------+---------------------------------+