Remove curly brackets around sample text in BBCode documentation

**Your Godot version:**
4.4.1

**Issue description:**
All the tag examples use curly brackets (these things `{}`).

> Makes `{text}` use the bold (or bold italics) font of `RichTextLabel`.
> `[b]{text}[/b]`

This will confuse people and they will type the curly brackets into their RichTextLabel's text properties as well. I think either there should be a big yellow warning note telling to not type in the curly brackets, or they should just be removed.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/tutorials/ui/bbcode_in_richtextlabel.html