🐛 FIX: fixing chapters numbering

[choldgraf]

This fixes the numbering for some chapters when we have a top-level file w/ numbered: true

[codecov]

Codecov Report

Merging #829 into master will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #829      +/-   ##
==========================================
- Coverage   84.84%   84.77%   -0.07%     
==========================================
  Files           8        8              
  Lines         660      657       -3     
==========================================
- Hits          560      557       -3     
  Misses        100      100              

Flag	Coverage Δ
#pytests	84.77% <100.00%> (-0.07%)	⬇️

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Impacted Files	Coverage Δ
jupyter_book/toc.py	94.44% <100.00%> (-0.11%)	⬇️

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