Correct the documented type for the $w parameter of block_core_navigation_add_directives_to_submenu()

[johnbillion]

What?

This correct the docblock for block_core_navigation_add_directives_to_submenu() which incorrectly documents the $w parameter as a string.

[jorgefilipecosta]

LGTM 👍

[aristath]

@johnbillion This PR was stale and there was a conflict with trunk.
I rebased it, so once tests pass we'll be able to merge it 👍

[github-actions]

Flaky tests detected in 9e4a2be.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/6455731108
📝 Reported issues:

• [Flaky Test] should insert elements from multiple completers in a single block #47916 in /test/e2e/specs/editor/various/autocomplete-and-mentions.spec.js
• [Flaky Test] should partially select with shift + click (@webkit) #53524 in /test/e2e/specs/editor/various/multi-block-selection.spec.js