Update link to DoD DevSecOps Reference Design 2.0

Lt Col Viola reported the link in docs/understanding-bigbang/README.md for Reference Design below is outdated:

## Value add gained by using BigBang

* Compliant with the [DoD DevSecOps Reference Architecture Design](https://dodcio.defense.gov/Portals/0/Documents/DoD%20Enterprise%20DevSecOps%20Reference%20Design%20v1.0_Public%20Release.pdf)

Should reference the new version (2.0) that is available.

New link to use: https://dodcio.defense.gov/Portals/0/Documents/Library/DoDRefDesignCloudGithub.pdf?ver=zXJ_uO5LfouVaysHo5Ejsw%3D%3D