Small changes to SQL persist README

Thanks for the detailed instructions! I was able to run the examples & tests. Here are a few very small changes I noted along the way. I am also wondering if it would be okay if I make ```tests/persist/sql/README``` just refer to this file, to avoid maintaining duplicate versions of the instructions, and just add these lines in the section on testing:

```
Edit lib/opencog-test.conf, and change the TEST_DB_NAME, TEST_DB_USERNAME,
TEST_DB_PASSWD as appropriate. Then, run the unit tests:

   $ ./tests/persist/sql/BasicSaveUTest
   $ ./tests/persist/sql/PersistUTest
```