Update doc: no spaces allowed in the json ID

[kpereyra]

I found that if there is a space in the id, it causes the close button ( class="close-button-remove") to fail. If there is a space in the id, the callout box (div class="tg-modal timeglider-ev-modal ui-widget-content ui-draggable" etc) appears in the upper left of the timeline and cannot be closed or dragged.

Fast fix is to update this piece of the documentation:

This needs to be unique relative to other timelines being loaded. To be safe, it should have a structure like "com.orgname.timeline-123" in case its ever loaded with timelines from other sources.