Fix wiki docs to send the page slug url encoded

What does this MR do?

The wiki docs contain a small error. When the page slug is passed in the URL, it needs to be URL encoded, but we don't say that.

In this MR we're updating the docs to explicitly say that the slug must be URL encoded.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Refs #328678 (closed)