doc: clarify text around MVCC example query

Reported-by: marlene.brandstaetter@cargonet.software

Discussion: https://postgr.es/m/167765529052.987840.12345375075704447735@wrigleys.postgresql.org

Backpatch-through: master

doc/src/sgml/mvcc.sgml

@@ -417,8 +417,8 @@
     does not see effects of those commands on other rows in the database.
     This behavior makes Read Committed mode unsuitable for commands that
     involve complex search conditions; however, it is just right for simpler
-    cases.  For example, consider updating bank balances with transactions
-    like:
+    cases.  For example, consider transferring $100 from one account
+    to another:
 
 <screen>
 BEGIN;
@@ -427,8 +427,8 @@ UPDATE accounts SET balance = balance - 100.00 WHERE acctnum = 7534;
 COMMIT;
 </screen>
 
-    If two such transactions concurrently try to change the balance of account
-    12345, we clearly want the second transaction to start with the updated
+    If another transactions concurrently tries to change the balance of account
+    7534, we clearly want the second statement to start with the updated
     version of the account's row.  Because each command is affecting only a
     predetermined row, letting it see the updated version of the row does
     not create any troublesome inconsistency.