fix capitalization to allow proper access to 'access AJAX API' perm.

[jmcclelland]

Maybe this is platform dependent? On my dev instance the access AJAX API
permission to the path /civicrm/ajax/api4/X/X is always denied for anon
users because anon users are granted "access AJAX API" yet the check is
done against "access Ajax API".

This only really matters in CRM/Api4/Permission.php but to avoid any
confusion I'm changing it everywhere the string appears.

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[colemanw]

I can confirm that 'access AJAX API' is the correct capitalization as defined in CRM_Core_Permission