fix docstring rendering for xparse

[KristofferC]

Used to look like (broken list):

Now:

[codecov]

Codecov Report

Merging #126 (ad7e747) into main (fbc1dd8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #126   +/-   ##
=======================================
  Coverage   87.62%   87.62%           
=======================================
  Files           9        9           
  Lines        2312     2312           
=======================================
  Hits         2026     2026           
  Misses        286      286           

Impacted Files	Coverage Δ
src/Parsers.jl	94.24% <ø> (ø)

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[nickrobinson251]

Thanks!