Swap contents for HTMLString and MarkdownString in docs

[christianversloot]

Describe changes

The docs accidentally give a Markdown example for HTMLString and an HTML example for MarkdownString. This PR fixes that.

* zenml.types.HTMLString for strings in HTML format, e.g., "# Header\nSome text",
* zenml.types.MarkdownString for strings in Markdown format, e.g., "<h1>Header</h1>Some text",

Pre-requisites

Please ensure you have done the following:

• I have read the CONTRIBUTING.md document.
• If my change requires a change to docs, I have updated the documentation accordingly.
• If I have added an integration, I have updated the integrations table and the corresponding website section.
• I have added tests to cover my changes.
• I have based my new branch on develop and the open PR is targeting develop. If your branch wasn't based on develop read Contribution guide on rebasing branch to develop.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other (add details above)

[strickvl]

Good catch. Thanks for the PR, @christianversloot!