Deployment instructions on www.elastic.co/downloads/ecctl don't support default macOS shell (zsh)

## Readiness Checklist

- [x] I am running the latest version
- [x] I checked the documentation and found no answer
- [x] I checked to make sure that this issue has not already been filed
- [x] I am reporting the issue to the correct repository (for multi-repository projects)

## Current/Expected Behavior


I followed these steps from www.elastic.co/downloads/ecctl today on macOS on 10.15.5 and run into this problem:

```
tony@tony-mbp ~ % source <(ecctl generate completions)
complete:13: command not found: compdef
tony@tony-mbp ~ % bash
bash-3.2$ source <(ecctl generate completions)
bash-3.2$ echo $?
0
```
The issue is [zsh is now the default shell on macOS](https://support.apple.com/en-us/HT208050). This works fine on bash.