fix(search): use arxiv shortcode in arxiv_class example

[thostetler]

• Replace long-form arxiv class name in the arxiv_class search term example with the short code format (astro-ph.IM)

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[Copilot]

Pull request overview

Updates the SearchBar search-term help content to show arXiv categories using the canonical shortcode format, making the arxiv_class example align with typical query inputs.

Changes:

• Replaced the long-form arXiv class name example with the short code (astro-ph.IM) for arxiv_class.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.4%. Comparing base (bb93ce0) to head (7be21b2).

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #829   +/-   ##
======================================
  Coverage    62.4%   62.4%           
======================================
  Files         317     317           
  Lines       36565   36565           
  Branches     1670    1671    +1     
======================================
  Hits        22816   22816           
  Misses      13711   13711           
  Partials       38      38           

Files with missing lines	Coverage Δ
src/components/SearchBar/models.ts	100.0% <100.0%> (ø)

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