docs(examples) 🎨: Fix dayClassName specificity issue

[balajis-qb]

Problem
This fix is related to a CSS specificity issue in the docsite example "Custom day class name". In that example even though the dayClassName is applied properly, it's not applied in the UI due to CSS Specificity issue.

Changes

• Increased the specificity of the CSS selector to apply the dayClassName style to the day element
• No functional changes

Screenshots

Contribution checklist

• I have followed the contributing guidelines.
• I have formatted my code with Prettier and checked for linting issues with ESLint for code readability.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.31%. Comparing base (644f2f5) to head (27de959).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6194   +/-   ##
=======================================
  Coverage   99.31%   99.31%           
=======================================
  Files          30       30           
  Lines        3810     3810           
  Branches     1639     1659   +20     
=======================================
  Hits         3784     3784           
  Misses         25       25           
  Partials        1        1           

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