Minor improvements to transaction manager README.

A simple SELECT is handled by PortalRunSelect, not ProcessQuery.  Also,
the previous indentation was unclear: change it so that a deeper level
of indentation indicates that the outer function calls the inner one.

Stas Kelvich

src/backend/access/transam/README

@@ -66,7 +66,7 @@ sequence:
      \  CommitTransactionCommand;
 
     /   StartTransactionCommand;
-2) /        ProcessQuery;               << SELECT ...
+2) /    PortalRunSelect;                << SELECT ...
    \    CommitTransactionCommand;
     \       CommandCounterIncrement;