doc: fix missing quotation in example code

[horsewin]

This pull request includes minor corrections to the presets/sortable/sortable-context.md file. The changes fix typographical errors in the items arrays within SortableContext examples.

Typographical corrections:

• presets/sortable/sortable-context.md: Corrected the items arrays by adding missing quotation marks in the nested SortableContext examples. [1] [2]