Correct exercises results and fix typos

09. Ordering your results with ORDER BY/CORRECTIONS.md

09. Ordering your results with ORDER BY/EXERCISES.md

...regate functions AVG COUNT MIN MAX and SUM/CORRECTIONS.md

...ggregate functions AVG COUNT MIN MAX and SUM/EXERCISES.md

12. Subqueries/CORRECTIONS.md

12. Subqueries/EXERCISES.md

18. Date and Time/CORRECTIONS.md

18. Date and Time/EXERCISES.md

SIB-SQL_for_data_science.pdf