Fix typo in Cauchy sequence definition

[wyf027]

Fixes #427.

Summary

• Correct the definition of a function tending to 0 so the formula refers to the introduced function h.
• Replace |f(n)| with |h(n)| in content/sets-functions-relations/arithmetization/cauchy.tex.

Validation

• git diff --check
• Verified the surrounding LaTeX source introduces h : \Nat \to \Rat immediately before the corrected formula.

Note: I did not run a PDF build locally because this machine does not have latexmk or pdflatex installed.