🐛 FIX: fixing toctree spacing bug

[choldgraf]

makes sure there's enough space for the added toctrees

[codecov]

Codecov Report

Merging #836 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #836   +/-   ##
=======================================
  Coverage   84.77%   84.77%           
=======================================
  Files           8        8           
  Lines         657      657           
=======================================
  Hits          557      557           
  Misses        100      100           

Flag	Coverage Δ
#pytests	84.77% <100.00%> (ø)

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

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