Fix typo: tnx -> txn (#4063)

Author: andreyaksenov

doc/concepts/atomic/txn_mode_default.rst

@@ -4,7 +4,7 @@ Transaction mode: default
 ===========================
 
 By default, Tarantool does not allow :ref:`"yielding" <app-yields>` inside a :ref:`memtx <engines-chapter>` 
-transaction and the :ref:`transaction manager <txn_mode_mvcc-tnx-manager>` is disabled. This allows fast 
+transaction and the :ref:`transaction manager <txn_mode_mvcc-txn-manager>` is disabled. This allows fast
 atomic transactions without conflicts, but brings some limitations:
 
 *   You cannot use :ref:`interactive transactions <txn_mode_interactive_transaction>`.

doc/concepts/atomic/txn_mode_mvcc.rst

@@ -18,7 +18,7 @@ but has a different implementation.
 
     Currently, you cannot use several different storage engines within one transaction.
 
-..  _txn_mode_mvcc-tnx-manager:
+..  _txn_mode_mvcc-txn-manager:
 
 Transaction manager
 -------------------
@@ -97,7 +97,7 @@ You can set an isolation level for a specific transaction in its ``box.begin()``
 
 ..  code-block:: lua
 
-    box.begin({ tnx_isolation = 'best-effort' })
+    box.begin({ txn_isolation = 'best-effort' })
 
 In this case, you can also use the ``default`` option. It sets the transaction's isolation level
 to the one set in ``box.cfg``.

doc/concepts/data_model/indexes.rst

@@ -260,7 +260,7 @@ RTREE index can accept two types of ``distance`` functions: ``euclid`` and ``man
 ..  warning::
 
     Currently, the :ref:`isolation level <transaction_model_levels>` of RTREE indexes
-    in :ref:`MVCC transaction mode <txn_mode_mvcc-tnx-manager>` is *read-committed* (not *serializable*, as stated).
+    in :ref:`MVCC transaction mode <txn_mode_mvcc-txn-manager>` is *read-committed* (not *serializable*, as stated).
     If a transaction uses these indexes, it can read committed or confirmed data (depending on the isolation level).
     However, the indexes are subject to different anomalies that can make them unserializable.
 
@@ -443,7 +443,7 @@ attributes.
 ..  warning::
 
     Currently, the :ref:`isolation level <transaction_model_levels>` of BITSET indexes
-    in :ref:`MVCC transaction mode <txn_mode_mvcc-tnx-manager>` is *read-committed* (not *serializable*, as stated).
+    in :ref:`MVCC transaction mode <txn_mode_mvcc-txn-manager>` is *read-committed* (not *serializable*, as stated).
     If a transaction uses these indexes, it can read committed or confirmed data (depending on the isolation level).
     However, the indexes are subject to different anomalies that can make them unserializable.