pySCG: CWE-89 image formatting wonky, need correct license quote for XKCD using CC-BY-2.5

[myteron]

We officially quote CC-BY-4.0 but external content such as XKCD has CC-BY-2.5.

TODO:

• Update main pySCG readme.md to quote something like: "images from XKCD are licensed under CC-BY-NC-2.5 and are noted as such" similar to what is done in secure_software_development_fundamentals.md
• in CWE-89 README.md fix formatting
• in CWE-89 README.md adopt image quotation from secure_software_development_fundamentals.md

Example image label below image:

Heartbleed Explained. Retrieved from xkcd, licensed under CC-BY-NC-2.5