Update cFS training package link

[skliper]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I reviewed the cFS README.md file to see if the feature is in the major future work.
• I performed a cursory search to see if the feature request is relevant, not redundant, nor in conflict with other tickets.

Is your feature request related to a problem? Please describe.
New slide package available at https://ntrs.nasa.gov/citations/20210022378

Describe the solution you'd like
Update README

Describe alternatives you've considered
None

Additional context
None

Requester Info
Jacob Hageman - NASA/GSFC