[Lesson 03] Fix typo in variable names in text.
[Lesson 06] Fix typo in function names in text.
[Lesson 09] Fix typo in variable name in text.
[Lesson 10] Typo in function name and order of arguments in function define_panels().

Order of the arguments in function define_panels() has been changed
to handle a default value (40) for the number of panels.
[Lesson 11] Add text between code cells.

Add some text between code cells when we are computing
the surface pressure coefficient.