Issue #3: Fix Typos in Lecture #9

[zhanghenglai]

This pull request takes care of Issue #3 by correction some typos in the **notes files for lecture #9. This includes:

• fixing the navigation links in README.md,
• specifying code snippets as PHP for some of the sample codes,
• correcting spelling mistakes, such as "Boolen" to "Boolean" and "exectured" to "executed",
• removing unnecessary apostrophes, such as "let's" to "lets" and "it's" to "its", and
• making other minor typographical and stylistic improvements.

@yurm04