small mvcc.sqml improvents

Here are two small changes to mvcc.sgml:

1. typo ('are' for 'at')

2. Change title of Table 13.1 to say 'Standard SQL' instead of just 'SQL', to make immediately
clear that the table does /not/ show the levels in postgresql, but the levels as described in the
SQL standard.  (the postgres levels are described in the text below the table)


Thanks,


Erik Rijkers