chore: fix uint256_addmod doc

[trocher]

What I did

Fixed the displaying of the doc after I messed it up in #3146

How I did it

How to verify it

Commit message

chore: fix uint256_addmod doc

Description for the changelog

Fix uint256_addmod doc

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[codecov-commenter]

Codecov Report

Merging #3300 (e68cc4b) into master (187ab0e) will increase coverage by 0.00%.
The diff coverage is n/a.

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@@           Coverage Diff           @@
##           master    #3300   +/-   ##
=======================================
  Coverage   88.86%   88.87%           
=======================================
  Files          84       84           
  Lines       10599    10594    -5     
  Branches     2214     2211    -3     
=======================================
- Hits         9419     9415    -4     
  Misses        770      770           
+ Partials      410      409    -1     

Impacted Files	Coverage Δ
vyper/builtins/_utils.py	100.00% <0.00%> (ø)
vyper/builtins/functions.py	90.43% <0.00%> (ø)
vyper/codegen/core.py	86.74% <0.00%> (+0.04%)	⬆️
vyper/ast/folding.py	93.27% <0.00%> (+0.53%)	⬆️

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