Incorrect Download command at https://www.calendarserver.org homepage

The correct download command at https://www.calendarserver.org should include the prefix `release/` in the branch name:
```
git clone -b release/CalendarServer-X.Y https://github.com/apple/ccs-calendarserver.git
```

E.g.,
```
git clone -b release/CalendarServer-9.3 https://github.com/apple/ccs-calendarserver.git
```