Tables: Fix striped tables

[niqjohnson]

Striped tables weren't actually getting striped because of a slightly wrong class getting applied.

Changes

• The table__striped class that was being applied to striped tables is now the correct o-table__striped

How to test this PR

1. Add a table to a page in Wagtail and check the "striped rows" option.
2. Preview that table and make sure the table is striped as shown in the Design System.

Screenshots

Checklist

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