Fix link to builders markdown.

[jmcdice]

Missing an 's' in the 'builders.md' filename.

[codecov-commenter]

Codecov Report

Merging #949 (d6cc882) into main (1768c41) will increase coverage by 0.25%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #949      +/-   ##
==========================================
+ Coverage   69.50%   69.75%   +0.25%     
==========================================
  Files         120      120              
  Lines        5492     5475      -17     
==========================================
+ Hits         3817     3819       +2     
+ Misses       1304     1285      -19     
  Partials      371      371              

Impacted Files	Coverage Δ
pkg/apis/build/v1alpha1/register.go	0.00% <0.00%> (ø)
pkg/apis/build/v1alpha2/register.go	0.00% <0.00%> (ø)
.../apis/build/v1alpha1/cluster_builder_validation.go	88.88% <0.00%> (+1.38%)	⬆️
.../apis/build/v1alpha2/cluster_builder_validation.go	88.88% <0.00%> (+1.38%)	⬆️

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