admin/render_readmes: Simplify total_pages calculation

[Turbo87]

div_ceil() does pretty much the exact same thing 😅

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.01%. Comparing base (31c98b4) to head (1db6a88).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/bin/crates-admin/render_readmes.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9894   +/-   ##
=======================================
  Coverage   89.00%   89.01%           
=======================================
  Files         290      290           
  Lines       30095    30092    -3     
=======================================
- Hits        26786    26785    -1     
+ Misses       3309     3307    -2     

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