Change default items_per_page to 10

[danchamp]

Current sitewide search value is 12:

https://github.com/localgovdrupal/localgov_search/blob/d01f3fbe445751547bb9a9eb4019dbd9c18f880d/config/install/views.view.localgov_sitewide_search.yml#L46-L50

Suggest 10 is conventional and expected by most users (Events uses 20, Directories 10, News 10).