fix SQL example syntax in file comment - Mailing list pgsql-hackers

Hi,

Here's a trivial patch that fixes a comment in execProcNode.c

For archeological interest, that comment dates back to when it was
written in POSTQUEL... The cleanup in
a9b1ff4c1d699c8aa615397d47bb3071275c64ef changed RETRIEVE to SELECT, but
forgot to add a FROM clause :)

Cheers,
Jan