[TRIVIAL] Update link to whitepaper and remove dead link

[ChristianRamseier]

• Updated the link to the whitepaper
• Removed the dead link to the "Ripple Whitepapers & Reports" overview page

[codecov-io]

Codecov Report

Merging #2228 into develop will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #2228      +/-   ##
===========================================
- Coverage    70.07%   70.05%   -0.02%     
===========================================
  Files          689      689              
  Lines        50730    50730              
===========================================
- Hits         35550    35540      -10     
- Misses       15180    15190      +10

Impacted Files	Coverage Δ
src/ripple/basics/DecayingSample.h	77.77% <0%> (-8.34%)	⬇️
src/ripple/core/Stoppable.h	93.33% <0%> (-6.67%)	⬇️
src/ripple/app/ledger/PendingSaves.h	93.54% <0%> (-6.46%)	⬇️
src/ripple/protocol/impl/Serializer.cpp	69.96% <0%> (-0.35%)	⬇️
src/ripple/app/ledger/impl/LedgerMaster.cpp	46.41% <0%> (-0.23%)	⬇️
src/ripple/app/main/Application.cpp	60.37% <0%> (-0.12%)	⬇️

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[nbougalis]

Merged as 7a0fa31