The underlying content-management system, Plone, calls these URL parts -- which are file or folder names in traditional file systems -- "short names". To simplify content creation, the system usually creates these short names for you by replacing all the spaces with dashes and converting to all lower case.

You may fix the short name for an individual item by going to the folder containing it, choosing the "contents" view from the green content bar, checking the box beside the item and pressing the "rename" button.

If you find yourself doing this frequently, visit your "preferences" page (a link on the personal bar, next to "log out"), choose "Personal Preferences" and check the box for "Allow editing of Short Names" in the form. Then, a short name field will appear with every item creation form.