Fix links

[rpiazza]

related to #512

[codecov-commenter]

Codecov Report

Merging #517 (3a0297c) into master (638689c) will increase coverage by 0.63%.
The diff coverage is 14.28%.

@@            Coverage Diff             @@
##           master     #517      +/-   ##
==========================================
+ Coverage   86.56%   87.20%   +0.63%     
==========================================
  Files         153      153              
  Lines       18043    17911     -132     
==========================================
  Hits        15619    15619              
+ Misses       2424     2292     -132     

Impacted Files	Coverage Δ
stix2/v21/bundle.py	70.00% <ø> (ø)
stix2/v21/common.py	51.61% <ø> (ø)
stix2/v21/sdo.py	60.93% <ø> (ø)
stix2/v21/sro.py	57.89% <ø> (ø)
stix2/v21/vocab.py	0.00% <0.00%> (ø)
stix2/v21/observables.py	36.31% <33.33%> (ø)

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