Minor example fixes in DM

All,

Section 2.4 of DM needs the following corrections:
http://www.w3.org/TR/rdb-direct-mapping/#no-pk

1) From the first row of the table, the following triple is missing: _:a
<Tweets#tweeter> "7"

2) From the second row of the table, the current triple:  _:b
<Tweets#tweeter> <People/ID-7> is incorrect. It should be _:b
<Tweets#tweeter> "7"

3) From the second row of the table, the following triple is missing: _:b
<Tweets#ref-tweeter> <People/ID-7>

I'm guessing we can do these minor changes before we go to PR.

Juan