Change Manage Sites capitalisation in the User menu

## Bug Description

Minor UI issue on the User menu. We have sentence case for the other menu option, and we should change `Manage Sites` to `Manage sites` so that it is consistent.

## Screenshots

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## Acceptance criteria

* `Manage Sites` label is updated to `Manage sites` in the user menu.

## Implementation brief

- [ ] In `assets/js/components/UserMenu/index.js`
- Update the label to `Manage sites`

### Test Coverage

- Fix the related failing tests and stories.

## QA Brief

*

## Changelog entry

*

Start in assets/js/components/UserMenu/index.js and inspect the related failing tests and stories for the User menu label. Confirm the displayed text uses sentence case, update the affected expectations or stories, and run those related checks to verify that the menu shows “Manage sites”.

Written by the indexing model from the issue text.