Fix broken link in the docstring of Thunk

[devmotion]

Fixes #559.

[nickrobinson251]

thanks!

[codecov-commenter]

Codecov Report

Merging #560 (9ce1220) into main (117910b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #560   +/-   ##
=======================================
  Coverage   93.02%   93.02%           
=======================================
  Files          15       15           
  Lines         860      860           
=======================================
  Hits          800      800           
  Misses         60       60           

Impacted Files	Coverage Δ
src/tangent_types/thunks.jl	95.37% <ø> (ø)

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