refactor(block-kit): remove nested rich_text block example in favor of standalone elements (#248)

Author: zimeg

block-kit/README.md

@@ -21,7 +21,6 @@ Read the [docs](https://docs.slack.dev/block-kit/) to learn concepts behind thes
 - **[Input](https://docs.slack.dev/reference/block-kit/blocks/input-block)**: Collects information from users via elements. [Implementation](./src/blocks/input.py).
 - **[Markdown](https://docs.slack.dev/reference/block-kit/blocks/markdown-block)**: Displays formatted markdown. [Implementation](./src/blocks/markdown.py).
 - **[Plan](https://docs.slack.dev/reference/block-kit/blocks/plan-block)**: Displays a collection of related tasks. [Implementation](./src/blocks/plan.py).
-- **[Rich text](https://docs.slack.dev/reference/block-kit/blocks/rich-text-block)**: Displays formatted, structured representation of text. [Implementation](./src/blocks/rich_text.py).
 - **[Section](https://docs.slack.dev/reference/block-kit/blocks/section-block)**: Displays text, possibly alongside elements. [Implementation](./src/blocks/section.py).
 - **[Table](https://docs.slack.dev/reference/block-kit/blocks/table-block)**: Displays structured information in a table. [Implementation](./src/blocks/table.py).
 - **[Task card](https://docs.slack.dev/reference/block-kit/blocks/task-card-block)**: Displays a single task, representing a single action. [Implementation](./src/blocks/task_card.py).