docs: added Lesson 3 Python challenge and other fixes

[tdeknecht]

• Adding a challenge to Python lesson 3 to close out this issue: https://github.com/StateFarm-STEM/hablogger/issues/116
• Fixed and issue where Lesson 2 source was accidentally uploaded as the challenge code. Replaced the code with the originally intended source for this lesson
• Adjusted the challenge in Lesson 2 to be more obvious and similar in its location and format to the new challenge introduced in Lesson 3
• Adjusted the file locations for images and videos in Lesson 2 to be consistent with Lesson 1. This same model will be applied to future lessons as Challenges are added
• Minor syntax or grammatical fixes

[pankonen]

@tdeknecht do you mind putting a template out somewhere on structure, format, pic layouts, etc of lessons so others can follow that when they pickup issues and knock out other challenges?

[tdeknecht]

@tdeknecht do you mind putting a template out somewhere on structure, format, pic layouts, etc of lessons so others can follow that when they pickup issues and knock out other challenges?

Yeah, I can do that. I opened this issue based on the suggestion: https://github.com/StateFarm-STEM/hablogger/issues/133