Fix the Markdown example; fix a typo

[kurtmckee]

In addition, indent the code block so it's a part of list item 3.

Markdown supports guarding a fenced code block with more than 3 backticks, which allows you to document Markdown code using Markdown code:

Old

Code

```mermaid graph LR Apples --> Bannana ```

Rendering

\`\`\`mermaid
graph LR
Apples --> Bannana
\`\`\`

New

Code

````markdown
```mermaid
graph LR
Apples --> Bananas

Rendering

```mermaid
graph LR
Apples --> Bananas