Summary

Format the go get installation command in the README using a fenced Markdown code block.

The previous installation example included the shell prompt ($), which may lead users to copy it directly into the terminal and encounter errors such as command not found: $.

This change updates the installation section to display only the executable command inside a fenced bash code block for better readability and easier copy-paste.

Changes

• Replace indented command formatting with a fenced Markdown code block
• Remove the $ shell prompt from the installation example
• Improve installation instructions readability

Before

## Installing

### *go get*

    $ go get -u github.com/slack-go/slack

After

## Installing

### *go get*

```bash
go get -u github.com/slack-go/slack

## Validation
- [x] Documentation change only
- [x] No API changes
- [ ] Run `make pr-prep`