chore(docs): use master link of NEP-539

[tinyfoxy]

since NEP-539 was merged, it's time to use master link of NEP-539 in the docs to render better reading experience. :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.61%. Comparing base (810e820) to head (c8f64ea).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12183      +/-   ##
==========================================
+ Coverage   71.59%   71.61%   +0.01%     
==========================================
  Files         824      824              
  Lines      165507   165507              
  Branches   165507   165507              
==========================================
+ Hits       118496   118522      +26     
+ Misses      41882    41858      -24     
+ Partials     5129     5127       -2     

Flag	Coverage Δ
backward-compatibility	0.17% <ø> (ø)
db-migration	0.17% <ø> (ø)
genesis-check	1.25% <ø> (ø)
integration-tests	38.73% <ø> (+0.02%)	⬆️
linux	71.40% <ø> (+0.01%)	⬆️
linux-nightly	71.19% <ø> (+0.01%)	⬆️
macos	54.10% <ø> (-0.08%)	⬇️
pytests	1.57% <ø> (ø)
sanity-checks	1.38% <ø> (ø)
unittests	65.35% <ø> (+0.01%)	⬆️
upgradability	0.21% <ø> (ø)

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