another mvcc.sgml typo

*** a/doc/src/sgml/mvcc.sgml
--- b/doc/src/sgml/mvcc.sgml
***************
*** 604,610 **** ERROR:  could not serialize access due to read/write dependencies among transact
      Consistent use of Serializable transactions can simplify development.
      The guarantee that any set of concurrent serializable transactions will
      have the same effect as if they were run one at a time means that if
!     you can demonstrate that a singe transaction, as written, will do the
      right thing when run by itself, you can have confidence that it will
      do the right thing in any mix of serializable transactions, even without
      any information about what those other transactions might do.  It is
--- 604,610 ----
      Consistent use of Serializable transactions can simplify development.
      The guarantee that any set of concurrent serializable transactions will
      have the same effect as if they were run one at a time means that if
!     you can demonstrate that a single transaction, as written, will do the
      right thing when run by itself, you can have confidence that it will
      do the right thing in any mix of serializable transactions, even without
      any information about what those other transactions might do.  It is