.github: fix automerge

[mmcloughlin]

Use html_url not number.

[codecov-commenter]

Codecov Report

Merging #273 (4b12116) into master (b04c1d7) will decrease coverage by 0.33%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #273      +/-   ##
==========================================
- Coverage   76.04%   75.70%   -0.34%     
==========================================
  Files          65       65              
  Lines       20591    20591              
==========================================
- Hits        15659    15589      -70     
- Misses       4854     4928      +74     
+ Partials       78       74       -4     

Flag	Coverage Δ
integration	11.84% <ø> (ø)
unittests	72.82% <ø> (-0.35%)	⬇️

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Impacted Files	Coverage Δ
examples/md5x16/md5x16.go	0.00% <0.00%> (-86.42%)	⬇️

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