JSON Schemas: fix menu_icon property in Options Pages schema

[priethor]

What

Fixes a bug in the menu_icon property that came up while adding Options Pages abilities in #273, as it had the wrong type.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 10.98%. Comparing base (34a448e) to head (192d758).

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #274      +/-   ##
============================================
- Coverage     10.98%   10.98%   -0.01%     
  Complexity     4288     4288              
============================================
  Files           266      266              
  Lines         33159    33159              
============================================
- Hits           3644     3642       -2     
- Misses        29515    29517       +2     

Flag	Coverage Δ
e2e	29.55% <ø> (-0.03%)	⬇️
phpunit	6.08% <ø> (ø)

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