Fixes struct example

[milancermak]

What I did

As mentioned #2422, the struct example in the docs is slightly wrong. Here's the fix.

Cute Animal Picture

[charles-cooper]

LGTM, thank you

[codecov-commenter]

Codecov Report

Merging #2425 (07fef11) into master (ed59064) will decrease coverage by 0.86%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2425      +/-   ##
==========================================
- Coverage   85.47%   84.61%   -0.87%     
==========================================
  Files          91       91              
  Lines        9027     9027              
  Branches     2151     2151              
==========================================
- Hits         7716     7638      -78     
- Misses        806      882      +76     
- Partials      505      507       +2     

Impacted Files	Coverage Δ
vyper/semantics/types/value/numeric.py	80.64% <0.00%> (-5.38%)	⬇️
vyper/builtin_functions/functions.py	83.53% <0.00%> (-4.93%)	⬇️
vyper/old_codegen/expr.py	75.52% <0.00%> (-4.59%)	⬇️
vyper/ast/nodes.py	92.12% <0.00%> (-1.47%)	⬇️

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