Update SELECTOR_REGEX for GranularMarkings

[emmanvg]

Fixes #406

[codecov-commenter]

Codecov Report

Merging #407 into master will decrease coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #407      +/-   ##
==========================================
- Coverage   98.43%   98.31%   -0.12%     
==========================================
  Files         127      128       +1     
  Lines       14652    14394     -258     
==========================================
- Hits        14423    14152     -271     
- Misses        229      242      +13     

Impacted Files	Coverage Δ
stix2/properties.py	97.89% <100.00%> (-0.53%)	⬇️
stix2/test/v20/test_granular_markings.py	100.00% <100.00%> (ø)
stix2/test/v21/test_granular_markings.py	100.00% <100.00%> (ø)
stix2/canonicalization/Canonicalize.py	59.03% <0.00%> (-2.01%)	⬇️
stix2/base.py	96.75% <0.00%> (-1.15%)	⬇️
stix2/utils.py	96.27% <0.00%> (-0.94%)	⬇️
stix2/test/v21/test_parsing.py	89.65% <0.00%> (-0.67%)	⬇️
stix2/datastore/filesystem.py	94.21% <0.00%> (-0.10%)	⬇️
stix2/test/v20/test_datastore_filesystem.py	99.19% <0.00%> (-0.02%)	⬇️
... and 67 more

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[clenk]

LGTM, thanks @emmanvg!